Having a Crossfire capable board, I'd say do ATI 5770 in Crossfire over upgrading to the GTX 460. Just switching from 1 5770 to 1 460 isn't going to be a massive leap. Spending $150 on an ATI 5770 (vs $200 to $220 on GTX 460) would give you Crossfire, which would outperform a single GTX 460.
You'd be gaming at ATI 5850 (or better) levels that way.
Which means you can game @ 1920x1080 with 60FPS or better in many titles.