I swore off of ATI hardware because of Linux drivers. Had 2x9600, 9700, X850XT and X1800XT. The s-video TV out on Linux for the 9 series was always hoplessly broken and never fixed. Fine, it's old hardware (even in 2005) and these days it's all about HDMI. X850XT and X1800XT were fine to use in dumb framebuffer mode, but running compiz together with OpenGL apps caused problems. The installer eventually got spiffy but missed files left by previous installers and caused plenty of issues starting with coredumps and ending with graphical glitches. Didn't play well with 64 bit without tweaking, moving and symbolic linking post-install.
To my knowledge not a single ATI driver (whether proprietary or open) accelerates video playback under Linux in any meaningful way (read: more than motion compensation). Contrast this to NV, where even the 8400 card can significantly accelerate video playback with common HD codecs.
But the biggest showstopper was game performance. Stuttering, occasional awful frame rates, lockups and texture rendering problems under Wine. Replaced with an NV card and got smooth sailing with gaming, desktop and video playback for going on 2 years now. Don't get me wrong, the 5XXX series looks real attractive. But I'll be reading the Linux forums at Rage3D to see if it's at all usable before I go red again.