MomotheMonkey7950's are going for around 200 now. Great price/performance ratio ATM. Who made your psu? A 600W shitty psu won't do you too well. Also, I'd wait a few days for AMD new cards to come out and look at benchmarks. But a 7950 would probably be your best bet right now.
I was going to respond to this, but this guy's post pretty much sums up my ideas.