^You just suggested that AMD is better yet said to go with a nVidia GPU when a HD4850/4870 is the current best bang for buck GPU out there......
Nice contradiction.....
Just to be honest to the OP, for gaming it truly depends. Some newer games are taking advantage of quads. Most still do better on higher clocked duals.
For a quad I always suggest the Q6600 until either Core i5/Phenom II is out or Core i7 920 drops in price.
For dual core on a budget any of the Athlon X2s or a nice Core 2 E7300. The Core 2 E7000 series OCs pretty well.
For video card, I suggest the ATI HD4850. Best bang for your buck really.
Memory, you want 4GB these days. Corsair is the best but Patriot has some decent RAM for a budget.
For HDDs, always do a RAID 0 with 2 drives. 500GB is around $50-60 bucks now so its pretty cheap.
For the mobo, Asus is always the best for me. A bit pricier even on their lower end mobos but usually because they come with nice features and great support.