I know a basic amount of information on pcs and I wanted to know if the build I made on PartPicker can be improved upon. Am I spending to much? Can components be improved for similar or lower prices? I'm looking for a budget of <2000
You need to change the AIO to a 280mm Minimum for the 9900K. I would go with Min 650W PSU. The PSU is not something to cheap out on. I would at least go with a Gold rated PSU but that's just my 2 cents. You need to make sure any case you go with supports a 280MM AIO
Yes, ATX standards are ATX standards, that case supports any ATX, mATX or mITX motherboard.
No, there's absolutely no issues with power supplies with this case, in fact the only cases that do have issues are non-ATX cases like SFF, SFX, some mITX etc, the really slim, small, tiny stuff. I've never heard of any ATX psu having any issues in an ATX mid-tower other than maybe blocking a bottom fan port which most don't use anyway.
I7-9700k is an 8 core/8 thread cpu. (think like an old i5 that's 4c/4t). The i9 9900k is an 8 core/16 thread cpu. (that's like the old i7 version of the i5 + hyperthreading). Is the i7 9700k good? Yep, sure is. It's the same as any other old i5, just one step down from the top of the line i7. Intel just bumped up the i numbers to differentiate from the 6c/6t i5 9600k with its 2 less cores.
The real question is how long will that last. Windows 10 and many games don't use more than 8Gb of ram, but quite a few do so 16Gb is universally recommended. Starwars will be releasing its latest game sooner or later. Minimum 8Gb, no surprise there, but recommended is now 32Gb of ram. Surprise. Just 6 short years ago, quad core cpus ruled budget oriented gaming. Now, about obsolete for any new title. 6c/6t cpus will be next. So just how long before 8c/8t start running into trouble with high % usage. The hyperthreaded 9900k will be absolutely the last cpu of recent generations to become obsolete. It's going to outlast them all. That's the difference.
More cores are only good if you use them. As far as the "how long will an 8C CPU stay relevant...hard to say, but I'd imagine its >5 years out. A LOT of other things can/will change in regards to PCs in that period of time.
Even the best games these days are only using between 4 and 5 threads. If you're running a lot of other things in the background, like streaming (if enough people care to watch you play) etc, then you can eat up an extra core or two. It's still the most efficient for game devs to offload as much game rendering to the GPU as possible.
Ultimately, the i7-9700K isn't much/any sacrifice compared to the 9900K when it comes to straight gaming and is pretty unanimously a chart-topper. Here's a couple graphs using FarCry 5, a pretty CPU-intensive game. Suffice to say the 9700K = 8700K (close enough).
FC5 with same in-game settings as above, while streaming 1080p 60fps, 6Mbps video bitrate.
In terms of pricing, PCPartpicker is an easy tool to pull a bunch of components together, but it won't give you the best prices. For that, you'll want to subscribe to newegg sales flyers, check at slickdeals.net, etc etc. Also, at this point, you might as well wait until the week of Black Friday to purchase.