There are many different video adapters available and choosing one that is well supported can sometimes be a challenge. Bill Farrow of the Triangle Linux Users Group (TriLUG) provides a very concise summary of three companies video adapters and his experiences.
Intel is an active contributer to Xorg, they provide GPL drivers for their chipsets and developers who work on improving Xorg.
ATI provides a proprietary driver (fglrx) for their graphics chips, and they now release their docs to Xorg developers which has lead[sic] to better GPL drivers (radeon).
NVidia only provides a proprietary driver that works very well, except when upgrading your Linux OS. There is an open source 3d driver (vouveau) being developed through reverse engineering.
I am running all three. Nvidia on my MythTV box so that I get h/w video playback acceleration, ATI at work with multiple monitors, and Intel on my quad core desktop.
If you want really fast 3D graphics and don’t care about running proprietary code at kernel level, go with Nvidia. If you want open source code, good performance, community support, go with Intel or ATI.