Uh, a few months ago i would have recomended the 4870, cuase it was alomst 40$ cheaper than the 5770 on newegg. But now, the prices have shifted around to the point where they are equal, the 5770 is even 5$ cheaper. So get the 5770 at the current prices. If they were to lower the 4870 back to 130$, and the 5770 rasied to 170$, I would get a 4870.
Figure the power savings is not only while Gaming -- IF you are using the computer for browsing the net and posting on forums etc. the Idle power draw of the 5770 over the 260 and 4870 will also make a small difference even if you are not gaming that much.