I highly recommend FALC0N's build, except that you may want a GTX 560 for light gaming and for the rare CUDA app. If not, then just go with a 6770 or a 7750, although you should check that it is GDDR5.
I can't even believe people will suggest getting a more expensive build with a $100 cheaper CPU and WORSE GPU. Also, PhysX is next to useless(it's just eye-candy).
TL;DR GET OPTION A.
Any of the HD 6xxx(lower than the 6850, faster than the 6450), or get the HD 7750. You also get the GT 6xx cards, but they are also a bit slow. I recommend the 7750 in case there is a slight chance that you want to game.