Unless you're truly suffering gaming wise, I'd hang on a couple months. The GTX 260 is a nice upgrade from the 8800GTS 512. That's what I did originally. But, with the 58xx series just released, and nVidia scrambling to release their 300 series, it's a very good time to sit back and watch.
The 58xx series seems to perform really well (5770 is okay I guess, but nothing outstanding from what I've seen). Once nVidia releases something, everyone's prices may come down a little due to competition.
However, you might start seeing price reductions on last generation cards just to get rid of them. For example, NewEgg did a $15 off 1 day coupon code for the MSI GTX 260 card earlier this week.
ATI did price the 5770 well though, as a good mainstream GPU. Comparable to GTX 260 performance I think, and has DX11 (which will happen in the future).