As for graphic cards, it is a little bit slower for Nvidia, slower with AMD, but anyway \\ the fact is that most AAA games are released on console too, and even half my graphic card has the power of more than a Xbox 360 and a playstation 3 together. Also every game that is multiplatforme and get a release on the playstation MUST be openGL (or in fact the proprietary playstation specs, ripped off openGL ES, a "simplified" openGL). Sure the port is not free of work, but with some preparation, can be relatively painless.
And the fact that Linux is more diversified, and thus harder to develop on, lies mostly. The majority of changes on the kernel are made with retrocompatibility in mind (just look he maillist of Wayland to see how sacro-saint it is), so any software made for an old version will work in the new (can't say that for win XP --> 7). Plus, a lot of people are on the latest version, or have all up-to-date (statistically, it doesn't appear well, since a lot of servers use "old" but safe software). So developing with the latest kernel in mind is safe, want to be even moar saffer, distribute your game with his own set of driver, Windows games do that since a while (do you want to install directX X.XX?).