If you're looking purely for a higher average framerate from a single card then the best advice is probably to wait. Problem is that your dual GTX460s will be delivering comparable performance to a HD5970/GTX580, and that's already near the top of what's out. The GTX680 would give some gain, but probably not enough to justify the cost.
Really, the only way you could get significantly more performance than what you already have is use a pair of more powerful cards. You could get a major average framerate boost then, but depends whether you're looking to get away from SLI and use a single card or are happy to stick with SLI. If it's the latter then a pair of GTX660s would serve you very well. If it's the former then I'd wait for GTX770, GTX780 etc next year.