You might take a look at m0n0wall. it's a lightweight BSD firewall/router distro.
I run mine on an old HP point of sale terminal with an 8mb flash card boot drive. almost completely web administered, with only inital device setup required to be attached to the system.
A friend was using a linksys router and was bogging it with P2P, much as was mentioned above, which prompted him to try this.
Also, if you have a craving to play with non-x86 hardware, it's built for ARM processors as well, if I remember correctly. Another friend is running his router on a Soekris(sp?) single board computer.
http://m0n0.ch/wall/