for your kernel you are using and the flavor of processor that the kernel is.
IOW, I have kernel-image-2.6.9-1-k7 installed, I need kernel-headers-2.6.9-1-k7. kernel-headers-2.6.9-1 will tag along as the other just includes K7 specific stuff and the symlink.
So.
Assuming you have the Laptop with an Intel P4M Processor
apt-get install kernel-image-2.6.7-1-686 kernel-headers-2.6.7-1-686 kernel-build-(whatever version is required) kernel-tree-2.6.7
kernel-build and kernel-tree are actually built to make modules outside the kernel tree, but mainly for Debian packages. Although it does work for some modules that don't know about Debian at all, just that they are simple.