Are you using a self-compiled kernel? If yes, then it might be enough to
create a symlink /usr/src/linux which points to the kernel-source-2.6.8
directory. (At least that was sufficient to get it working for me.)
If you are using a stock Debian kernel (and your output of "uname -a"
seems to suggest that you do), then you need to install the appropriate
kernel-headers package. (If you did not compile the kernel yourself then
the kernel source directory will not contain the information which the
nvidia installer needs.)
You could also try the Debian package for the legacy nvidia driver,
"nvidia-kernel-legacy-2.6-686".
Regards,
Florian