Early ATI Linux drivers were indeed a pain.
They have improved quite a bit recently.
nVidia has always had better drivers on all platforms including Linux however.
Both ATI and nVidia make great hardware, but to this day nVidia has better compatibility and better drivers on all platforms.
On Linux and *BSD I would recommend nVidia over ATI.
On windoze both are great, nVidia drivers are arguably slightly better.
nVidia also kicks butt in OpenGL which is particularly handy in Linux and *BSD.
Semper Fi Carry^H^H^H^H^H Linux on!