The HD5770 will be a decent upgrade but not huge, something along the lines of 35% but it will also give you DX11 compatibility and use significantly less power, especially at idle.
If you can save up and snag the HD5850 it is a good idea. It would be approximately twice as powerful as your 8800GTX at stock and can overclock a huge amount(30-40%) on top of that. If you want a card that could last you 3 or so years like your old one then that is the way to go. I'd recommend this one in particular;http://www.legitreviews.com/article/1273/1/