What New Desktop Should I Buy?

The Alienware you posted seems good, although you could probably turn most of those specs down if you wanted to save money at the expense of only having amazing gaming performance instead of insane gaming performance.

The way I'd approach this is:

• You need enough memory to never run out and no more than that. Memory speed doesn't matter in any practical way. This is an easy choice since 16 GB is slightly too low for a lot of things and 32 GB is usually more than you need. Presumably you can open the resource monitor on your current computer and have a decent idea of how much memory you need. Don't bother future-proofing this since memory usage seems to have plateaued for a while.

• Gaming performance is going to almost entirely determined by your GPU, but the entire RTX series is so ridiculously overpowered that the 3060 Ti is good enough for most purposes, including some single-monitor 4K gaming. Any GPU that has good gaming performance will be ridiculously overpowered for displaying browser tabs on three monitors.

• One additional consideration is that you should get an Nvidia graphics card since you're likely to want to run accelerated machine learning code. A consideration for higher-end cards is that some of them have more memory, which increases the size of models you can accelerate.

• Within the range of CPU options where a pre-built will have 32+ GB of memory and an RTX card, it probably doesn't matter which one you pick. I'd just get the cheapest i7 or Ryzen that they offer since they're all going to be overkill. I don't recommend the Lenovo option you posted since it has a Threadripper, which is really good CPU for a use-case that you don't have (saturating 20+ threads). You'd be better served with a similar-price CPU with better single-threaded performance. (It's possible that loading massive spreadsheets could use a bunch of threads, but I doubt that Google Sheets actually does)

• The power supply just needs to provide enough power for your CPU + graphics card + a little breathing room. Note that the RTX 3070 + i7-12700KF only use about 500 W together so a 750 W power supply is plenty.

So I think the base model I'd recommend to you is this XPS tower:

https://www.dell.com/en-us/shop/desktop-computers/xps-desktop/spd/xps-8950-desktop/xd8950adl10h?view=configurations&configurationid=1a2e2625-a3b7-401a-8db7-c093f562219b

... and then turn up the memory / GPU if that doesn't seem good enough. The Alienware you posted is similar and would be worth it if you want liquid cooling and don't mind being forced to upgrade to at least an RTX 3070.

You might also consider getting an older desktop with an RTX card, since an older CPU and memory really wouldn't affect the performance much.

I have a Lenovo P620.  It's a bit noisy, but not intolerable.  I haven't had any hardware problems.  Some people have reported problems after installing a Lenovo firmware update, so maybe don't do that (I haven't).

It may not be compatible with all graphics cards that you would think it would be - I found that a GTX 780 didn't work for no identifiable reason. Possibly, they've disallowed all but "professional" GPUs. (An NVIDIA K40c did work, and it has the same "Kepler" architecture as the GTX 780.) I currently have one AMD WX2100 GPU (for actual display use) and three NVIDIA A4000 GPUs (for compute).  I had had four A4000 GPUs, for which there are four suitable 16x PCIe 4 slots, but found that they wouldn't all run at full speed due to cooling issues (this is a bit of a problem even with just three).  This may say more about the A4000 than the P620, however.  The 1000 Watt power supply of the P620 is useful if you want lots of GPU power.

The AMD Threadripper Pro processor is powerful, but single-thread performance is a little bit below the top Intel offerings. It has eight memory channels, so if you populate all eight DIMM slots with memory cards, you can get a lot of memory bandwidth.  Note, however, that the NUMA memory hierarchy makes fine-grained communication amongst processor cores slower than on some Intel machines.

I bought the P620 to use for ML and other intensive computational tasks. It may be the most powerful affordable option for that.  For general desktop use with a bit of gaming, there are probably better choices.

As is generally the case for all compute vendors, it will be a lot cheaper to buy a minimal system and then install memory, SSDs, HDDs, and GPUs from a third-party source, though that of course is more of a hassle and could produce compatibility problems.

Linus Tech Tips has a secret shopper series where they (anonymously) order several prebuilts and test them out.  They find that they vary greatly in quality, with Dell (which owns Alienware) being the absolute worst.  iBUYPOWER takes the prize, with boutique brands Maingear and Origin PC doing well, but you will pay a price premium.

Gamers Nexus also reviews some prebuilts and finds a lot of them horrifically terrible, especially Alienware.  Like, really, it's not that hard to build your own PC; how can these prebuilts be so incredibly incompetently built?

For those who aren't scared of the prospect, I highly recommend building your own.  You may have to spend some time researching (I'm happy to recommend specific parts), but it's a process I enjoy, and it makes the computer feel much more "yours."

I bought my last computer before the pandemic so perhaps the supply chain stuff makes this harder, but I just bought the exact parts of a recommended build from https://pcpartpicker.com/. I had never built a PC before and had little idea what to choose. The resulting computer turned out well and I didn't have any problems. If you are not a hardware enthusiast but do decide to take the DIY route, using one of these standard builds is about as painless as DIY can get: https://pcpartpicker.com/guide/. 

Though if you account for the cost of your labor (it took a few hours to put together), I imagine it is not worth it. 

1. Nvidia/Windows tends to not do well with 4 monitors, (this could apply specifically to 4 4k 144hz monitors). Had a bunch of reliability issues esp with returning from standby.    3 works great.  I've yet to try multiple gpus, I instead run two pc's.   If you care less about gaming I'd get the Nvidia a2000, It's basically a prebinned rtx 3060 with ecc, and lower power usage)  Likely would run 4 monitors at high framerates, but I have not tested it yet.  
2. Figure out if you are likely to notice a high framerate monitor, and if so get high framerate displays.  Your best bang for buck in 4k's (Assuming you care about latency and resolution and less about gaming or HDR) is the gigabyte m28u or m32. (28inch vs 32)  If you want smaller LG has a few, LGGN950 i believe.  Keep in mind heat output, 5 of these at 4k 144hz will draw about 200w of power.  
3. I'd get the 12900k or ks not the KF, so you get the added flexibility of the iGPU. KS is the prebinned intel OC'ed version of the K/KF.  
4. Your money is absolutely no object upgrade is an optane p5800x for your OS.  It's an enterprise drive with ridiculously low latency, makes your desktop usage much snappier.  I had a 905p previously, probably just as good in practice.
5. Quiet is all based on ambient temps and cooling.  Keep in mind modern processors do a great job of boosting their speed if they have thermal headroom.  Your best bang for buck option is to use an arctic liquid freezer ii AIO, as big as you are comfortable with.   High end is custom water cooling loops.  

(My current main desktop is a 12900ks, 128gb ram, 3090, p5800x.  previous was a 5950x)

Feel free to msg me if you want to discuss further.  

I've been using a lenovo ideaCentre G5 14IMB05 for two years now and it's been very powerful and useful for ~900 euros.

The current one in Lenovo store seems lower-end than the one I bought (my specs are RTX 2060, i5 processor, ), but there's probably something awesome within that space. With respect to your specificationa:

• windows 11
• Quiet fan, I often don't hear it. Has an ambiance sound when I'm using it intensely but it doesn't feel loud
• Has 4+ port for monitor displays
• Never noticed any lag (Often opening several vs-code instances and running services at the same time)

I've also often used the Alienware you linked thanks to a friend of mine. It really is awesome, but unless you often do machine learning/very high graphics with lots of visual details and frames gaming, I don't think it is that worth it.

Wish you the best with your search ^^

My wife and I have always built our own desktops, as component choice (speed/brand/specs) for the SSD and RAM matter a bit for performance.  A few years ago, that stopped making any sense - CPU and GPU dominate performance, and most of the time you're better off with MORE ram rather than FASTER ram at a given price point.  Also, pre-builts are far more detailed with their specs than they used to be, and supply chain oddities have made parts more expensive for consumers than for bigger orgs.

Last year, my wife agonized a bit over building her rig anew, and went with https://www.ekfluidgaming.com/, primarily because they actually had GPUs available at only slight markups, but also because they're one of few reputable builders who do watercooled CPU and GPU, which really does cut down on the noise (the fans are still noticeable in a quiet room, but it's not the jet engine of modern multi-fan systems).  After a shipping snafu (broken unit arrived), which they replaced without issue, but delayed things about 2 weeks, she's been delighted with it. 

For me, I'm finding myself on the couch or in my front room with cats and sunshine fairly often, and I'm likely not to replace my aging desktop with another full computer.  I plan to get a beefier gaming laptop, and use a docking station for my office(s - I have two rooms in my house with monitors and good keyboards set up).