I'd avoid ATI like the plague!! ATI does not support their products long. (This also is true in windows) The next time you go to update your Xorg server, you may no longer be able to use your ATI card.
I have old computers 8 years+ and the nvidia embedded card still work with modern linux distros. Still have hardware acceleration.
Nvidia support is far superior than ATI now. 5 years ago I would have recommend ATI but not anymore.
I mostly own AMD CPUS computers with NVIDIA videocards. They alll run fine on linux. My Intel netbook runs fine too. Amd cpus, better bang for my $$.