Personally I would probably go with a single GTX 570 because I prefer to have one stronger card over two weaker ones, but some of the other setups would perform better.
The 6850 is a weaker card than the 5850 (yes, that's how AMD decided to name them) so switching to two of those would be pointless. So you can rule out that option.
Switching to two 460s would be better than your current setup but probably only about equal to having two 5850s in CF (the 5850 is a stronger card but the 460 scales a bit better). So I would rule out that option too.
So that leaves you with the option of buying a stronger single card (you also have the 6970 to consider as this is similarly priced to the 570) or getting a second 5850 for CF. The two setups would perform similarly, with the 5850s possibly performing a little higher. But then you have drivers, heat, power consumption, and PCI lanes to think about. Of course, a second 5850 would be a lot cheaper. So it's basically up to you to decide which one you'd prefer.