I'm using 256.35 and have no problems with TwinView. It's on the lowest performance level (50/135 on my card) most of the time (I don't use compositing in KDE).
I also didn't see crashes or freezes when using VDPAU or OpenGL (then it switches to higher performance levels without a problem).
I'm using Fedora 13 and my card is a Palit GTX 460 Sonic.
For VDPAU you can test http://www.nvnews.net/vbulletin/showthread.php?t=133465