Not even worth it in my opinion. The 5770 is a really good card, just use MSI Afterburner and OC the Core Clock and Mem Clock and your good to go. The 5770 is also relativly new so there is no need to upgrade. If you say you were going to upgrade to something like the 5850 or 5870 which would be a much larger improvement sure. But as of now, you don't need to since the performance gap is so small, sorta. Also, as a side note a good OC on my 5850 gave me a 2000 point increase of graphical performance on 3DMark Vantage, so you really don't need to spend 200 more dollars. Instead if you have the money, buy another 5770. You'll save cash and the Crossfire performance is equivelent to that of a 5870, therefore in theory it is able to run almost any game in Highest of the high. My 5850 oc'd runs practically all games on Highest, except a bit laggy with anything more than 4x AA.
Just buy another 5770 the scaling from one to two 5770's is AWESOME!
If your board supports crossfire them 2 x 5770s is a no brainer if not I don't think any upgrade is great for the price as anything thats more than just a bit better costs alot and it could even be cheaper to get a new MB and a second 5770. The GTX 460 is good but as an upgrade from a 5770 I would only consider it if you are desperate for just a bit more.