I have a 670 ( About a year old now, I think ) - And it's great for gaming. Haven't had performance issues in any games since I got it.
The only reason I'd avoid AMD at the moment is because I like PhysX. This can be worked around though, with a cheap GT610 or something to supplement your AMD card, and run the PhysX calculations on the side. It takes a little hacking, but it works pretty well, and you can save some money.