Why go for a 570 ? It will only give you a VERY MINOR gain in fps , or no gain at all ... i would wait for 600 series , for what you have is enough , i think ..
The GTX 460 is a beast card alone, in SLI there outstanding.
I would have to say a 570 would not be close to an upgrade, they might even be equal. It would probably take 580's in SLI to give you an actual boost.
You could also wait for the 590's. I don't know when they will be avalible, or how long supply will last.
All in all, the only benefit would be power consumption for getting a 570, and if thats what your looking for, go for it.