Probably like the difference between a 6800ultra and 7800gt, since i owned both the diff is around 30%, with lots of smoke and stuff like that on the screen the 7800gt does much better otherwise the improovement is something u would expect with 4 more pipelines.
If your buying a new one 7800gt is a better choice but costs more, otherwise if u already own a 6800gt, gs or ultra, or the ati equvalent, not much point in upgraiding just wait for the next gen cards. Only reason i have my 7800gt is cause my ultra died and i got it as replacement.