The specs for the 6770 and the 5770 look the same but charts seem to say that the 5770 performs a little bit better.
And we can't really tell you how much better it will be, since it depends what you are playing.
In the charts I look at, for an average FPS rating of games played on maximum setting, your 9800 GT would get around 16 FPS.
The 6770 would get you around 49 FPS.
The 5770 can get you around 52 FPS.