4850 for the price. You can get one for 150 USD now. The 4870 is still too expensive compared to 4850 but very good. The newer ATI that will come out in a month or two will be much better:
http://www.techpowerup.com/78670/AMD_Sets_RV775_to_Chase_Down_G200b?.html
(RV770 = 4850 and 4870, RV775 = new replacement models)
http://www.techpowerup.com/78670/AMD_Sets_RV775_to_Chase_Down_G200b?.html
(RV770 = 4850 and 4870, RV775 = new replacement models)