I'm 99% sure that both Nvidia and ATI periodically release proprietary drivers for use with Linux distros. Also, there are open source versions of drivers for both Nvidia and ATI cards. How they all compare in stability and performance, I'm not sure, but I do know that proprietary drivers tend to do better performance-wise and that for advanced graphical effects you must use the proprietary drivers.
That said, if you want to run ATI cards on Ubuntu you should probably be looking for both kinds of drivers, open source and proprietary, and try out each. See which one works better for you. More than likely, the proprietary one will as it comes straight from the company and so it's tested, up to date, stable, etc. Not saying open source drivers aren't, but I would think they're not as good and take a while to get just right.
I don't mean to sound like an ass, but I've gotten every ATI card I've ever come across working fine under ubuntu. There is one exception, a FireGL-X1 and the issues I had with that I also had under windows... in fact it ran more stable under ubuntu.
If you can't get your card working you need to "lurk moar" and "learn to ubuntu".