IMO avoid ATI if you want to use Linux. Nvidia support is much better. I have used very old ATI cards with Linux though, things may have improved in the meantime. Still, my experience is that Nvidia's drivers are much more regularly updated, and I've never had any problems with using Nvidia cards and drivers under Linux.
Alternatively, see if you can maybe strike a deal, I have an understanding with my local computer shop that I can try out a card and see if it works, if not I can return and exchange. But then, they are quite positive toward Linux and open source experimentation.