If your budget is $300, I'd say go with the GeForce 760, you can find one of those around $249 USD. The 7990 is way above your price range, the 6770 is discontinued, and the 760 is substantially faster than the 640.
I'm assuming with that AMD FX8350 you've got a power supply that's 500W or more? If so, should be fine for that card.