I'm currently using an ATI card with Kubuntu, I used Envy to download and install the drivers. It just worked.
I was quite surprised, after reading lots and lots of horror stories about ATI drivers, open vs closed source and of course Nvidia is great and ATi is bad comments I installed Envy and 5 minutes later my ATI card was working, xorg.conf had been updated and I could play Nexuiz / Urban Terror without any problems.
I had to turn the Nexuiz graphics settings down a lot otherwise I would get a lot of random freezes, especially when a bot appeared on screen.