So... are you asking about the GTX 260
or GTX 460
? there is a bit of difference between the two.
The 8800 GT is (currently) #30 on the Graphics Performance List here on TF. The GTX 260
comes in at #17, while the GTX 260
Core 216 comes in at #14. The GTX 460
is currently not on the list, but should fall between the GTX 470 and the HD5850 (I think), or possibly a notch or two lower, so call it 8.5 for error's sake.
Either way both beat the 8800 quite soundly. A single 460 is an awesome card. By the time you are ready to buy a second one something newer and beter will be available, so don't plan TOO strongly on going with two of them in SLI.
Trust me on that.