Even with pre-builts, I think it’s not great because some of those inflated prices have caused knock-on increases (or at least prices that have refused to decrease over time for the same spec tier) you pay when buying anything in stock. Some of the worst of the RAM price increases are now starting to come back down, while GPUs continue to be pretty all over the place (but at least we are now at a point where stock exists and it’s not more expensive than the launch prices from 2017, or even 2016).
In terms of future-proofing, now isn’t a bad time for a CPU. AMD are extremely competitive with two gens of Ryzen out (the second being an incremental refresh more interesting for build-your-own due to bundling ok coolers with every model) and Intel have been forced to respond with 50% more cores in their mainstream desktop models (looking like it’ll be 100% more cores than a couple of years ago by the end of this year - some suggest soon so maybe a Summer refresh). The Intel single-core performance (and being the established platform everyone optimises for) means they’re generally a bit ahead in games but it’s far from definitive (and tweaking a single setting can flip who leads) and only AMD will sell you a cheaper 8-core/16-thread option and an outright budget 6-core/12-thread model (which have certain future-proof benefits running code when more engines are better at balancing loads on all cores). I’d definitely note that Intel 7x00 model CPUs with 4 cores are a bad buy (find at least an i5 or i7 8x00 with 6 cores if you’re not buying a Ryzen 5 or 7).
With RAM being relatively expensive right now (and console ports still building around at most 8GB for the game and even then no console uses that on the CPU end because they really want most to be VRAM for textures and frame buffers - consoles only having one pool for both) then 16GB is fine right now (and should give a couple of empty slots to upgrade in 2 years). One thing to check for a pre-built: two sticks - RAM today with these mainstream CPUs work on a pair of channels so you need at least two sticks of RAM for the full bandwidth.
For gaming, I’m doing quite ok with a 4K system and a GTX 1070 I got in 2016 (didn’t realise it at the time but buying at release turned out to be a very good move as crypto-nonsense has really messed with prices since then). Before that I was going after 1080p with a GTX 760 (which is no longer going to cut it, looking at various minimum specs being announced). This is extremely a case of “buy as much as you can get away with up to a point”. Over a certain point you’re paying a lot more for slight increases in performance, under a certain point then you’re getting a lot less performance for every dollar you don’t spend. See which companies have good prices, warranties, options, and availability (and even sales going on) and then see what’s available in terms of GPUs. Everything is kinda expecting a refresh (possibly delayed by crypto stuff meaning everyone punted their releases back) so now isn’t a great time to buy. But also everything is quite fast, it’s just not as cheap as it should be. AMD have ok mainstream models in the RX570/580 but note that these are a year old and they are basically warmed-over refreshes of the 470/480 they released two years ago. Meanwhile the sorta-equivalent GTX 1060 is basically unchanged from release two years ago except a mid-cycle VRAM speed boost (prices have not come down nearly as much as you’d expect because crypto have been eating stock for the last year). Above those cards you have a pick of AMD Vega 56/64 or nVidia 1070/1070 Ti/1080/1080Ti (the first three of those being all slightly different revisions of basically the same thing while the 1080 Ti was a monster when it came out in early 2017 and is still a monster today). They can quickly become expensive (so check reviews/benchmark comparison stuff for how much more you’re planning to spend vs what you expect to see in games).
I think most people now expect nVidia to release their 11 Series stuff in October (Computex is in 2 weeks so who knows! Maybe an announcement is very soon?) but maybe it happens sooner. AMD currently say not to expect new GPUs in 2018 (they may release new model numbers but they’ll be rebadging previous chips for OEMs who want to sell new numbers each year even if they’re the same actual chips as last year). As nVidia already sell the fastest GPUs, it seems like they’re not being forced to release anything new (despite most of these 10 Series cards being 2016 releases) but hopefully they aren’t also waiting until 2019 because scientific users have been buying completely new GPU designs for a while. We’ve even seen nVidia talking about Volta-only gaming uses at GDC despite none of the consumer GPUs being Volta models right now. Weird crypto-times.
I’d also think about storage. Right now you can buy unthinkably fast SSDs (sometimes saturating a 4x PCI-E 3.0 link so basically quarter of the max speed that the GPU uses to load all those textures to the VRAM) which has significant benefits for loading times in well-optimised games. But that’s not cheap and fast M.2 (the name of the fast connector) drives often ask “how small can I get away with?” We’re talking $100 at retail for 250GB, $200 for 500GB so that’s not an insignificant part of the total cost of a new system. And a single game can be over 100GB so you’ll want a second (slow traditional multi-TB) drive for media and backups of the games you’re not currently playing. I think it’s very worth it to put Windows 10 system files and a few games you’re currently playing onto a fast M.2 drive for the removal of that bottleneck, especially now Steam makes it extremely easy to shuffle installs around multiple drives. That might be something which is easier to buy after-market (and ensuring you get a properly fast drive) rather than taking the risk with whatever OEM options you’re being sold (eg Samsung make different drives for OEM vs retail and some are good but if you don’t know the exact model then sometimes you get something that’s far from cutting edge performance). Just grab the 2TB platter with the PC then plug in an SSD (assuming they give you a motherboard with an M.2 slot for it).