Best way to go is to build your own computer... for good speed make shure your ram FSB maches your cpu FBS, then get a 8mg cache hd, you can get aney size of a hd as long as it says 8mg cache on it, a nice video card Nvidia or AIT 128mg or higher, (There are also some realey good 64mg video cards out there to) A well known mother board bran like MSI, Gigabyte, or ASUS. And dont for get to go AMD. then your cooking with speed. twiking windows will also help.
I have a saying "
your computer runs as well as you take care of it"
Personley i got 3 computers. One i have thats my server hasent been formated in 5 years, never got a virous, or a crupted file problem. The new one im using now is build for gaming, and another one im working on needs a hd. Thats for my friends when thay come over thay have there own computer here to play on.
(Note: if your FBS on your cpu is higher then your ram FBS you will get a slower speed, or a unstable system "Blue Screen Of Dath". if your ram FBS is faster then your cpu FBS. lets say your cpu FBS is 266 and your ram FBS is 400, your not using 134 of the FBS. that shouldent hurt you pc but there is a 10% it will make problems. that and y spend the extra money.. A plus is that if you mother board chipset maches your video card bran is a real bounes to gaming 
Nvidia chipset or ATI)