all right, I got back to vanilla kernel 2.6.31, disabled some more kernel options and reinstalled the official driver release (185.xxx) and it now works really nice
The amazing thing is that even without the RT patch, I get an outstanding performance at very low latency, there is simply no glitches at all, even with composite effects (under KDE 4.3). I rotate the cube, flip through windows with slide effect, and ardour just works flawless at barely 1ms latency.
This is a real change from the onboard intel IGP which caused me a few xruns from time to time.
Back to nvidia, I am not sure what the problem is with the realtime patch and 3D performance but I will continue with the vanilla kernel for a while and see if I can ignore the RT patch.
I also tried vdpau with mplayer and it is spendid as well
Ah by the way, I have a dual monitor setup, so all the stuff I described takes place in a twinview arrangement.