Originally Posted by xazero
And, why does Firefox and KDE work just fine with Nouveau?
I could be wrong, but my bet is that because Nouveau's implementation is not as, "greedy" psycho as NVIDIA's for doing hardware acceleration. Namely, as long as you do not hit certain render paths, NVIDIA driver is faster and hardware implemented only, but if you hit certain ones (such as big pixmaps in Firefox and bits of KDE 4's drawing) then ugly things happen and the driver does a fair amount of software call backs which introduces excessive traffic on the PCI bus, this is a guess though.
Though, I have to admit I have really, really tried to like KDE4, and I cannot stand it. What I liked in KDE3 disappeared in KDE4. Every few months I try to make myself use KDE4, and the entire interface just bothers me... I used to love KDE over GNOME, but now KDE4 just, sighs... that is enough I think. I have not seen myself the performance issues people posted about KDE in GNOME, even with the useless compiz effects on.
Just to warn you: be prepared to be severely disappointed with GL under any card besides NVIDIA under Linux. If you do not do 3D graphics or the most you push your video card is Quake2, then it won't matter really.