Debian 3.1 aka Sarge ships with Linux kernel 2.6.8, which was released on 13-Aug-2004. This is not only outdated by itself, but if you want to have custom kernels with custom patches, most of them rely on recent kernels. This article summarizes the information gathered while installing a nfs4 enabled kernel, which consists of a 2.6.14 kernel with custom patches.
The following is based on an article on debian-administration.org by Jean-Chrstian de Rivaz. However, I did improve it a bit, so that you do not need any ugly dpkg --force-all options and no fiddling with dependencies and stuff in the dpkg database.
The whole procedure has been tested on i386 and amd64 with linux kernels 2.6.14 and 2.6.15-rc3.