well what resolution do you watn to game at and how much do you care about playing on medium/high/ultra?
if you want 1080p on high, you're looking at a 760. if you want 1008p on ultra, look at a 770
What's your budget? Adding another 460 you can get on ebay for $50 or so is your best bang for the buck.
That would put you on performance level near a 760