Your question states GTX 460 SLI or Single 560 Ti? So here is a little bench mark to show you the difference in the SLI vs a single 560 ti. Also the 460 vs 560 Ti in single cards No SLI. Hope this helps and good luck to you.
If you can sell the 460 and be happy with what you get for it then you would be getting an upgrade by getting the 560Ti and then getting another one later for SLI.
It would also depend on how long before you can get the second 560Ti because you can get the second 460 now and have the SLI setup now.