Is there a certain price range? Do you prefer AMD/Nividia/ATI?
There are alot of GPU's that will work, you just need to find one that suits you and doesn't bottleneck. Fore example, a GTX Titan would work, but the raw power of the thing would bottleneck from here to the moon and back.
then buy the 2gig 6950/7850/7870 and see how it goes. you don't need a motherboard and although the power supply is a little light for me, I think the system will still run okay.............. if not, then worry about spending the extra money. what is your monitor resolution?