There is a lot of hype concerning video cards. Here's the real story:
Chief characteristics:
FX5900
-big clock rate, and the resultant fill rate and memory bandwidth
-great 2D image quality
-very good drivers, application, and OS compatibility
-challanged by a few of the lastest DX9 games, but only in rare cases do serious consequences result
-faster by most other measures, including 3DMark03. My 5900 flashed to 5950 exceeds 7000; ATI's will not in my machine.
-a very nice solution if the occasional DX9 performance issue doesn't impact your application
ATI 9800
-moderate clock rate, but architectural advantages offset this
-very good 2D image quality
-decent drivers, spotty application and OS compatibilty beyon the most mainstream of considerations
-more adept at the DX9 games that give the 5900 problems
-a very nice solution if the occasional incompatibility does not impact your application
Certainly 9800s have relative strengths, but a 5900 (especially a cheap one) has virtue as well. After you've tried both you'll know which you really like better, as comparison with the forthcoming 6800 has shown they are really more alike than different after all.