Big difference is the 4850 can handle higher resolutions than the 8800/9800 without a huge performance hit. And then there is the story of AA
That depends on your definition of "huge performance hit," and what you consider typical AA. It can handle up to 4x AA at up to 1920x1200 without too much performance hit.http://www.anandtech.com/video/showdoc.aspx?i=3341&p=14
At $110 without counting in mail in rebates, the frame per dollar ratio for 8800gt is still higher.
Seriously, 4850 is a good all around card already without being best bang for the buck. You don't have to have it all. Let it go.