Well, contrary to popular belief, a processor is not as important as it used to be, so you can get away with a low-end dual-core if you have a decent graphics card. I only have a 2.13ghz processor, yet I can play Assassin's Creed just fine, owing to the greatness of my graphics card, in this case, a Geforce 8800GTS 512mb.
If you're looking for a decent gaming rig, try looking at different computer-building sites, then see if you can get the bits cheaper elsewhere.
Also, you won't get much use out of more than 2 gig of RAM, unless you have Vista. If nothing else, it's apparently built to make better use of RAM.