GTX770 would be nice, if playing 1080p..., This will be an excellent upgrade. You'll be able to run with full details and AA for any game with smooth video.
ASUS recommended(ASUS GTX770-DC2OC-2GD5 GeForce GTX 770 2GB 256-bit GDDR5 PCI Express 3.0 HDCP Ready SLI Support ).
De diference with GTX780 performance is negligible if you get some GTX770 EVGA superclocked version, it will perform very similar.