which should I buy

Solution
That's more than enough power for whatever you want to put in there. I'd say go with the GTX 760 if your budget is $300 US.

CraigN

Distinguished
I mean... the 7990 is better than the 6770...and the GTX 760 is better than the GT 640...

How about you give us some more information on the rest of your system? I mean, if you've got money to throw down for a 7990, and as long as you've read up on AMD's frame-pacing issues, go for it, but only if the rest of your system can keep up and not bottleneck it...
 

CraigN

Distinguished
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.