I say go for ATI 9800 if you have the cash. If not, then settle for something a little less then that. ATI seems to be whooping Nvidia's butt right now, and there's no showing any slowdown.
I heard HL2 officially supported Radeon 9800 at E3 or something. Goes to show the great leaps ATI has made, in chipsets as well as drivers, during its long rivalry with Nvidia.