I can only confirm that Nvidia works almost flawlessly in Linux; I grew tired in experimenting with ATI/Linux and bought the cheapest 9600GT-card I could find. A Gainward, @ ~70$:
1. glxgears -> 23000 fps+ against 550 fps w. ATI(software mesa only, HD4xxx)
2. All native Linux 3D-games run at blistering speed with the highest settings, even FligthGear runs smooth.
3. Did a comparison bench in UT2004 on the phoronix-test-suite against a HD4870 with fglrx(working), the 9600GT was average 5 times faster in all resolutions!
4. Steam installed without a hitch, automatically dled and installed HL2 at my command and I could immediately run it at the highest settings with 4xAA, 16xAF, HDR and reflections on. Totally smooth and with flawless IQ.
That is with a lowly 9600GT.
5. As an unexpected bonus, so does Nvclock work beatifully in trimming the fan duty cycle on the card. At a snap I could make it totally silent at a 25% fan duty cycle during desktop work, I do not need to trim it when I start a game, because the fan duty cycle automatically adjusts itself upwards when it's running 3D under load. Couldn't be better. Originally I was thinking about if I should smack an Arctic Cooling passive gpu-cooler on the card, but thanks to Nvclock there's no necessity
3D is in short working, thank's to Nvidia, I can imagine how beastlike a GTX260 or higher must be under Linux
I'm running 190.18 under 2.6.31-RC5, and at the moment I couldn't be happier. So much that I decided to pass on HD5870 for my main gaming rig and instead wait for the G300