I used a vanilla 2.6.15 - the i386 is compiled for 386 cpus, but you have a much more powerful cpu, why waste computing power for downward compatibility to such an old cpu? Do you plan taking the hdd out of your laptop and putting it into a 386?
The link to the gentoo-wiki for undervolting p-ms is already a howto that works perfectly on vanilla 2.6.15 kernels (nothing else was available on dapper or breezy when I installed the lappy) and is complete in almost every way I can think of.
Do the patch like explained there and make your own kernel. If you send me a pm with your emailadress I can send you my kernel-config so you don't need to find out everything useful yourself (you just have to change from build in graphics to ati). Of course I do not take any responsibility if this config does not work for you
(but at it is already the 14th version of my 2.6.15 kernelconfig I guess it's already quite good).
If you have problems following the wiki I suggest you write down what you did and what does not work after that.