Hello, I'm far from an expert on computers, hence why I'm posting here.
I've had a Gateway FX 6860 UR20P since March, and I've been wanting to upgrade my GPU. I haven't changed anything on this PC since I got it. The card can be from $400 down, and it will be used to play Planetside 2 prefferably on high, or BF3 on Ultra. I was looking at the GTX670 as it seems to run games quite well. I think it would last me a while for what it costs.
Side note: I also heard crossfire Sapphire 6950s would work better than the 670.