I'd say go with nVidia because of the next version of their drivers.
i think the current release is 61.72, but i've seen people gettion as much as 15 more fps in the HL2 VST with the 66.00 beta drivers. I think the next release will be 70.xx so its about halfway there
The question is do you really want to have to run HL2 in DX8.1 on your brand new card, or DX9 in and still with more FPS?
The same issue plagues many other DX9 games with FX cards.
if ur cpu is below 2.6ghz....got for the fradeon 9600XT. The 9800pro has too many pipelines for a cpu under 2.8ghz (or amd equivelant). But dont worry, 9600XT kicks butt, and u save 50 dollars