AMD's are actually underpowered for gaming. Their most powerful card is a six-core, which isn't applicable for gaming, and it is easily outperformed (even in applications where a six core would be ideal) by the low-end i5 2500K.

Not only this but Intel has better features like SSD caching, in which you can buy a cheap 40-80 gig SSD and use it as the cache for an HDD of any capacity, giving you a boost of HD speed of up to 80%.

Same deal with ATI's. Nvidia's architecture is superior so any ATi card is going to have to be pushed to its limit in order to compete.

ATi has a slight advantage in the 150-200$ range, but that's hardly gaming grade these days and if you're buying in that range then plan for your computer to enter obsolescence with the next batch of gaming consoles.