Don't buy a prebuilt. 99% of the time they are garbage and a total rip off.
CPU - 6 or 8 cores. anything less than 6 is severely outdated and anything over 8 is a waste of money. You could argue that most games are only really starting to make use of more than 4 cores but with the amount of 6 or 8 core cpus out now there is no reason to get 4. 6 cores will do fine but ideally go with 8. If you're building a PC for VR and getting a 144Hz monitor then you need a very good CPU. my recommendations:
6 core cpu - Ryzen 3600. Probably the best budget CPU out right now. Will destroy anything you throw at it.
8 core cpus - Ryzen 3700X or Intel 9900K/9900KS. Ryzen 3700X will do the job but if you want absolute gaming performance then nothing beats the Intel 9900K/9900KS. The difference between the K + KS is minimum and not worth the money. Get whatever is cheapest. The Intel will give you better higher frames which is crucial for VR/144Hz.
GPU - there are a LOT of gpus. AMD aren't really that great in the GPU department. You'll want an Nvidia card. a 1070 is roughly the same as a 1660 or 1660 Ti. I wouldn't recommend either those cards now the 1660 Super is out (it sits between both in terms of performance). After that GPU the next step is RTX gpus. I really don't need to say it but you should spend as much as you possibly can on your GPU.
RAM - no less than 16GB. i would recommend 32GB if you plan on using this PC for 3-5 years. 16GB is fine now but in the next couple years i reckon it will start to struggle. I used to have 16GB but games can easily use 10-13GB now so i upgraded to 32GB. any make is fine (corsair/g.skill are good). If you get an Intel cpu you can basically just throw in any RAM kit and forget about it. If you go Ryzen then you need FAST ram with low latency timings. 3200Mhz and CL16 is the minimum. Anything over 3600Mhz is a waste of money. The lower the latency (CL16 fore example) the better.
Storage - GET AN SSD. this is very important. even if you get a 250GB drive to install windows and programs on it. you can stick in a large capacity HDD for all your games. ideally you'd buy another SSD. i would highly recommend an NVMe SSD and not SATA3 SSD. sata3 isn't bad but it's an old standard that is starting to die out. all my drives on my PC are NVMe now except a large capacity HDD for backup. as for storage i highly recommend either Samsung (expensive) or WD (cheaper but still good performance).
PSU - please. do. not. cheap. out. on. your. psu. don't do it. so many people buy high end CPUs/GPUs and then power it with a crappy PSU that will end up just frying your entire PC. make sure it's 80% plus rated gold efficiency. I swear by EVGA PSUs. i have a 750W EVGA SuperNova G2 and it's rock solid.
CASE - get one that you like the look of. just of course make sure it fits everything lol. i mean don't buy a mini atx case with an ATX motherboard lol (not that you would but still...) other than that...i would just read reviews and see what ones are easiest to build in.
Motherboard - the main thing is pairing your motherboard with CPU. you get a Ryzen CPU you need an AM4 board. If you get a Ryzen 3700X you need an X470/X570/B450 board. If you get a 9900K you need a Z390. just make sure your motherboard has all the ports/connections you need. my last motherboard i just bought purely on how it looked lol.
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It's really overwhelming when you're new to PC building but just keep reading up and learning. If you have any more questions (you will...) or need help then we're here to help you