Please allow me to disagree with the rest of these guys with regards to linux support with ATI cards. XFree86 has supported ATI's cards for some time now, and unless your aim is to game on the system, ATI cards are actually better than nVidia cards in terms of features supported and such.
While it's true that in an official capacity ATI's driver support isn't there, the cards themselves have been supported for some time now through the
DRI project. Indeed, quite a few people I know use R9100/8500 cards for dual display purposes. I feel safe in saying having used both a Radeon 64MB VIVO in linux and a GeForce 3 Ti200 that nVidia's linux drivers are somewhat lacking in IQ.
IMO, there's no problem running a Radeon, unless you are afraid of installing the packaged DRI drivers, in which case, a Matrox card (for 2D quality) would probably suit you better.
That being said, if you do intend on gaming, then an nVidia card is the best way to go, no doubt about it.