which Video Card ?
when installing major linux distros RedHat, Mandrake, Suse, Debian, Slackware
which graphics card is commonly supported by these distros?
when installing any of these distros, which card is most likely to be detected and installed with proper drivers?
ATI and nVidia have good support. The ATI drivers can be a bit tricky but as long as you don't have the newest of the new cards there should be support. I don't know about other companies though...
As SiezedPropaganda says, nVidia has good support for linux, and an easy "setup", it's has become easier now (the previous drivers I installed were a pair of rpms, now it's just one file to run).
I haven't tried ATI my self but I've heard of others who tries to install drivers (he's apparently experienceing problems).
So, my recomendation is nVidia, and downlaoding (+installing) the latest drivers will make games playable (I think that is so for all gfx cards: you'll need t install the drivers before you get 3D acceleration.)
the best manufacturer is currently nvidia. that is if you want high performance 3d graphics. the open source graphics drivers suck for nvidia cards (nvidias fault) but the closed source official nvidia drivers work very well however if it buggers your system you will not have much support of cource. if you only need 2d acceleration then a cheaper card with open source drivers will be fine! i run suse linux and the drivers it installs are the open source ones so with suse (and i think all the other distros) you need to download about 3 MB of drivers from nvidia.com