If its for a windows machine to play games I'd go for the ati 9800. The entire FX5xxx series of cards are not really 100% DX9 compatible.
Nvidia cheats a bit and this results in some image quality loss, there has been quite some discussion about this, especially with games like far cry and HL2 in which you can see the differences quite clear.
The ati 9800 clearly has better image quality and is technologically a better product. In speed they are about equal, sometimes the ati 9800 takes the lead, sometimes the fx5900, but it's only by a small margin.
if the pick is between the ati 9800 or FX5900, I'd take the ati 9800 any day
