[Fingers]

The Radeon 9600 should be good enough as such, in fact the machine I used when programming the game had one of those cards.

Are you playing the game in a window or fullscreen? Are you using Vista's 3D accelerated interface? I don't have any first hand experience with Vista but I know it's pretty demanding on the video card. When running in a window it'll have to render both the OS and the game using the 3D card, and the 9600 may not be up to the task.

You could also try launching the game with the -verbose command line option and see what it reports about your video driver.