I have purchased a new sound card (SB Live 24-bit) and had to compile a new kernel (2.6.12) because my current kernel (2.6.10) didn't have driver support for it.
When I start KDE control centre, X seems to lock up. The keyboard does not respond so I can only reboot by cutting power. The mouse cursor still moves but nothing responds to clicks. The main window in control centre just displays as white.
I've traced the problem to the NVIDIA drivers. The 6629 drivers don't seem to build on kernels > 2.6.10 so I had to use 7167 ir 7667. These drivers cause the lock-up even in 2.6.10... So at the moment I have to use 2.6.10/6629 and no sound...
Problem solved. In xorg.conf "Device" section, set option "RenderAccel" to "false". Despite this, glxgears doesn't seem to be any slower than normal. Now running on 2.6.12 / NVIDIA 7667 with beautifully working sound.