I cannot really help you with Debian specific issues, please visit this for a Howto that addresses upgrading to the 2.6 kernel: http://home.comcast.net/~andrex/Debian-nVidia/
(there's a lot more to find on various forums, including this one).
You can check whether your kernel uses 4K stacks by checking your .config:
# grep CONFIG_4KSTACKS /usr/src/linux/.config
If it says something else, or it says nothing at all, or you have no .config, you will have to configure the kernel, compile and install it. (All this assuming your kernel sources *are* in /usr/src/linux, btw. ;-)