GPU: If you're going for the 3000 series of card, which with their pricing, you may as well at this point, you're definitely worth getting a CPU from this Gen.
As for why they're so much cheaper than the 2000 series cards, theirs a few things to consider.
1. RTX was new when it came out, so price of production was probably higher.
2 AMDs Big Navi is slowly catching up and with new consoles dropping at the same time, Nvidia have compeition.
It's also worth noting, the performance are nice on the 3080s, but it's not as some people will think. The 3080 is twice the power over the 2080, but when it comes to game performance and the extra benefits they gain, it's not 100% better. Instead of hitting 130 frames, you may get 150. (Game and engine dependant)
The big win for the 3000 series cards is the cost. They've been slashed in half, so you're getting a better GPU at half the price, but in game, it's not twice the performance of a 2080
CPU: Back to the CPU, either a i7-10700k or wait and see what AMDs new Zen 3 chips are going be like (The i9-10900k would probably be overkill at the price vs performance, and if I remember correctly, they benchmarked the i7-10700K vs an i9-9700K and the performance was almost equal at a much lower price. I am building my new machine soon and depending on what AMD do release, I'll either go with them, or with the 10700k
RAM: Because the cost of ram has dropped, I'd aim for 32gb at this point, but 16gb would more than cover what you need. Just make sure you get some decent speeds. I'd suggest looking at 3200+, but you'll need to overclock it in your bios
240hz. I assume you play a lot of FPS games.
I started off with a 165hz monitor. I made the switch to a 240hz and I honestly don't know if I know the difference. I'd have to drop my hz to do some comparisons, but I cant be bothered as theirs no need to drop lower.
Going from 30/60 of consoles to 144hz+ on PC is crazy different. I mess with a console now and its awful.
Different games will perform differently though. Apex is easy to run so I cap my 240 without a problem, yet something like Fallout 76 or The Division for example, that can easily drop to 80-140. Still playable, but you can sometimes tell, especially Fallout)
SSD is a must for gaming. Solid state drives retrieve data way quicker than the old HDD. Everything from booting windows, to loading screens in games are faster. It's always nice to get a decent HDD though as a secondary (Or in my case, my 8th storage unit on my PC - Not that I need it, I just collect storage over time and never fill it) storage to store basic stuff like pictures, music, documents etc is handy
Overclocking is ok. It's not necessarily needed though. You may squeeze a few extra frames in game by doing so. I think it really comes down to people wanting to push limits for fun. Also if you do want to overclock, definitely go with a watercooler.
Also mouse and keyboard. Do some research. I can offer some suggestions, but it's worth watching a guy called rocketjumpninja. He compares mouses, plays FPS games and he rates his best mice to use. I switched from what I thought was a great mouse, the Logitech G502 to a Cooler master MM710 and it was a game changer.
Also, buy a decent PSU
A few good people to watch on youtube are:
LinusTechTips
BitWit
GamersNexus - He goes way in depth and I find the videos a bit boring to be honest. The others keep it shorter, but he's still good
Also worth checking on the website pcpartpicker as you can build a machine and it will tell you if it has any compatibility issues
JaysTwoCents
This pretty much covers everything, but I'll add my thoughts as well.
GPU: Check out sites like
Userbenchmark which will give show you GPU-specific FPS rates of different games based on the settings you choose. I'd strongly suggest waiting for the 3000 series to drop - you get a far
superior peformance from a 3080 by paying a little more than you would for a 2070 super, while the 3070 would perhaps cost the same as the 2070 Super. The new RTX series also boasts of real-time
ray tracing, so that is another massive motivation. (If you're into deep learning as well, the boost in CUDA cores is also very significant)
CPU: Don't just stick to Intel, AMD has produced some really brilliant competition on this front. You could save some money here by sacrificing a little performance - look the Ryzen 7 and Ryzen 9 series (new entries underway) as well. But if you have the budget and want top of the line, nothing beats the i9 10900k as of now.
RAM: If you're going for 16gb, get 2 8gb cards, or if you're getting 32, get 2 16gb cards, 3200Mhz would be very helpful, but you can get away with 3000Mhz as well, depending on your use cases.
Refresh Rates: I personally don't think you'll notice the difference post 150-160 fps in any game. The initial jump till there is very noticeable, but after that the benefits taper off. For instance, I used to play Apex on my laptop at 20-30 fps (don't ask) and I barely got into Silver-I/Gold rankings. On a smurf account from a friend's PC, I jumped to high-Gold/Diamond on 120 fps. Unless you're really really good at the game, it's not worth going beyond a 144hz/165hz on a Full HD/1440p display.
SSD: Crucial, like mentioned above. Go for an NVME M2 1tb +
OC-ing like Igor said is a vanity exercise more often than not. I would struggle to think of why you'd need anything OC-ed if you have an i7/i9 10th gen on a RTX 3000 card with good RAM and SSD. Similarly, RGB lighting everywhere is another vanity expense that ends up costing a lot of $$. You don't need RGB cooling/RAM/fans/etc. per se - a fancy cabinet with good cooling and RGB fans is more than enough. But again, that's your call, so choose wisely because RGB adds a lot to your bill.
Once you have your basic build figured out - calculate what PSU you need
here. If your needs are 80W (say), go for a 100W PSU. Corsair CX has the whole range covered, iirc.
Invest a lot in your gaming chair, a good XL sized table pad with ample space for a good cutom DPI mouse (Razer/Logitech) and a keyboard which takes care of ghosting and has media controls. Check the recommended distance between screen (depending on size/aspect ratio) and your chair, and make sure you have bandwidth of >100mbps to actually make your hardware count - it's pointless having a 3090 on an i8 10th gen if you have 300ms ping on Apex.