keep in mind that when runing your os and computer programs like 3d art and stuff they use your systems ram mostly, but when playing a video game, it uses your video cards ram mostly... you can have 4gigs of ram on your pc with a sub standard video card and excpet to get crap, and vice versa 128mb of ram with a 128pro or a geforce 5700+ could probably pull it off.
I built my pc from "bleeding edge" stuff wich is the good stuff that just went below "leading edge" so there is a big price diffrence.
just got cheap ram 512mb of it and im running it in duel channel so its like 700mb of ram effectivly.
A geforce 5700 ultra wich has the super fast clock speed on it
a good overclocking mother board
and a pentium 4 2.8 ghz with the 1mb catch instead of the 512kb so its like 2x the size of ram on the chip (but that really doesnt make a big diffrence)
it runs fine even at max settings, only lag i get is sever lag not pc lag.
so depending on what kind of computer you have maybe just upgrade, or maybe buy a new one, i would tend to stay away from already built computers like dell or HP because they use specific parts that can not be upgraded or interchanged with standard stuff but only things they manufactor.
look for a computer fair or somthing, get a bare bones system as they call it, then go pick the parts you want and have your own machine thats just what you want.
hard drive wont matter so much, ram brand wont matter so much (unless you plan on over clocking), mother board wont matter so much... but dont be shy on the video card thats gonna be the big thing.