RAM: (Amount) (Speed) (Example: DDR3, 16GB, 1600mhz (1333mhz)) ( 1600mhz ddr3 ramsticks are just 1333mhz sticks that are overclocked already, If I recall correctly) (could be wrong, been a while since then)
GPU: (Brand) (Model) (Example: AMD Radeon, R9 390x) (Example: Nvidia GTX, 770) (Optional Example: GTX 1080, EVGA(Distributor/sub-brand), 8GB GDDR5X (VRAM)) (OE: GTX 1080TI, Gigabyte, 11GB 352bit GDDR5X)
HDD/SSD: Space is nice and all, but the speed of it is fairly important too, and connection type - For example, I should upgrade my HDD because they're one of the things I haven't been upgrading, and could stand to see improvements. I think If I recall right, I have a 160gb and 1tb 7200rpm (both) SATA HDD, Not sure if both are 6GB/s SATA, one should be, that I play games off of. (the 1TB) The RPM and Connection quality are important on lag sometimes, but most definitely on loading times. I still remember when I was playing APB Reloaded, the difference changing such things made... Had rather ridiculously long load times turn into loading screens that were like 1/100th their previous time.
Other things: (plenty of things, usually come with a computer but can also end up there from other games and websites, slow down/impact computer/game performance)
There's some other things I'm sure that are more important, but my mind suddenly started blanking.