9800GTX+ is equal to GTS 250 which is equal to 4850 (more or less, of course). Would you guys recommend a 5770 as an upgrade to a 4850? Probably not. It's a mostly horizontal movement, with only a piecemeal performance gain. The main benefits would be lower power consumption and DX11, but it's just too little of a performance boost to possibly recommend.
The 4890 would be the best performer in your price range, but with a Phenom I I'm not sure it is wise. I also have a PH1 and see no need to go above a 4850 with it without wasting the potential. Grab a 955BE CPU like recommended earlier. Then when the 5850 is at $200 or less (hopefully Nvidia competition will drive it down to there by the summer) grab one.