Try www.newegg.com It would also help if you let us know a little bit more info (such as socket type for the processor). Another website that's fairly popular is www.tigerdirect.com
Go to newegg.com, they have a lot of customer reviews that help me out a lot. Look on this forum, i'm always learning new things. If you ask someone what's the best stuff, i'm sure ou'll get different opinions. Like me, I prefer AMD and NVidia, but that's my opinion. Even though I have a P4 and ATI! (Just wanted to see how it was)