What is the rest of your rig? If you have a crap processor and low RAM Doom 3 will be almost unplayable even at 800*600 medium. Unless you turn off shadows and bump mapping. But that will make Doom 3 look like TOTAL POS.
If you have at least P4 2.4 and 1 gig dual channel RAM DDR PC3200 you can run it at 800*600 medium with shadows and bump mapping fine at around 30 fps except heavy gunfights.
Truthfully though if you're a Doom 3 enthusiast you should buy an Nvidia card as they perform better. But don't buy anything less than a 5900 and play on high 1024*768.
Honestly though with 9200 it won't be good and it will lag. Playable...yes. Tolerable...might be a different story.