I am currently using an OLD compaq deskpro which is like 90 Mhz with 16 megs ram.  I threw in a 500 mb harddrive, and 2 ethernet cards.  I installed Debian and got ipchains and ssh...  set your inside ehternet card for 10.0.0.1 or whatever network you prefer and there you have it!

Has not failed me yet!

-RJ