Establishing the root cause of a freeze

If you get hangs / freezes / crashes in your machine, the first thing is to establish the root cause of the problem. For this you must try to ssh into the target machine from another PC.
If you can ssh and restart X normally, the bug is probably in the X server or on the desktop environment (KDE).
If you can ssh but you cannot restart or kill the X server, try to look at the kernel logs (dmesg). There's a chance the issue is due to the nvidia kernel module.
If you cannot SSH at all, the issue is probably in some parts of the kernel not related to X / nvidia / video, and that's the hardest problems to solve. It could be a hardware issue too.

Kernel Configuration

You need to activate some options in the kernel configuration. Everything is under the section "Kernel Hacking".

If you run non GPL modules such as the nvidia drivers, it is very difficult to get a kernel to build with some debugging support, as it needs the modules to be GPL too. Most of the times, the system will refuse to boot.

This combination of keys (plus another one) allows to control the kernel even if X is completely crashed. You can output interesting information (this goes through the system logger though), or give the system some commands.
Note that you must activate this key in the kernel configuration; you can have it even with binary non GPL kernel modules.