iMac driver help

Hiya all. I just bought a iMac 20 inch. It has an ATI Radion 2600 graphics card with 512 mb of vram. I recently purchased and installed VMWare fusion, and loaded XP on. I'd watched videos on youtube of some stunning mac performance, and bought the orange box. To my frustration, Portal is running at an abysmal 2-3 fps. Worse then my old white macbook. I know something is wrong and my computer can run this game fine. If anybody can help me, or send me the link to drivers or direct x versions which will make it run better, it would be appreciated!