The GTX460 is roughly equivalent to the GTX280, but uses less power and runs cooler. I made that change myself, from a 280 to a 460, and it was deifnitely worth it for me as the 280 ran quite warm and sucked down the juice.
Compared to the 260 it's a nice jump in performance and should still use a little less power.
It really depends what your priorities are, how much the jump would cost you, etc - for me, my 460 cost less than I sold my 280 for so it was a no-brainer.
