I would also suggest a hd6850 with what you told us, but we will need more specs. PSU, budget, res. what games? Some games like Nv, some AMD.
I have a corsair cx500 PSU for the price $200 or less, preferably under $175. My max monitor resolution is 1600 X 900 60 Hz and I'll be playing Minecraft, Crysis 2, Bulletstorm, basically the newer games. and ill be doing Let's Plays using fraps if that matters.