I'm also running this with an ATI Radeon 9800 and I can't even get into the game because the video is too choppy. it takes 10 secs just to click on something. According to the "System Analysis", this video card is supposed to be supported, but I'm not seeing any evidence of this.
P4 3.0 Ghz
2 Gigs Ram
ATI 9800 Pro (128 Meg Ram)
So, I don't believe this is a "performance" issue, considering I can run Doom 3 with video options cranked up a bit.