I followed these instructions to get the ATI driver (a Sapphire 9800Pro) installed on my system:
http://www.nforcershq.com/forum/viewtopic.php?t=46624&highlight=ati
I don't know if you have an NForce2 mobo or not, but it might work the same without the 'modprobe nvidia-agp' line.
Unfortunately, I found that the drivers for ATI on linux are not all that great. For example, in Never Winter Nights, there is a strange 'sparkle' effect throughout the game. Very annoying. UT2004 seems to work ok except for the occasional unexpected crashing ;( .
I switched to an NVidia FX5900 and it is quite a simple and stable setup.
Hopefully the link will work better for you than it did for me.
