The GeForce GTX 460 SLI setup that you currently run is definitely ample enough for today's games, but there are also benefits of upgrading, too. Let's say you were to grab yourself a new GEFORCE GTX 670, this would give you a reduced power consumption compared to your SLI setup.
Secondly, it would give better performance as the single GPU can go right up to 100% usage, compared to some games, where you might see maybe 75-80% usage per GPU, not maximizing your setup. Thirdly, you would drop any current issues you experience with SLI, as there are some issues, with some games with multi-GPU use.
For pure performance, you're looking at an improvement, that's for sure. Two GTX 460s are equal to around a single GeForce GTX 480, and a GEFORCE GTX 670 is around ~30-40% faster than the single GTX 480, all while using less power, and producing less heat and noise. I say go for it. It'll also give you the chance to upgrade to GTX 670 SLI in the near future, too.