I'd recommend a 3rd server box. It will need 2 or 3 NICs in it that are linux compatable. There's probably a gui somewhere to turn on routing. One nic to The World, one nic to a DMZ (for your other 2 boxes), one nic to internal network. DMZ is sorta optional.
Do a bare install of Debian, add your favorite text editor, and get gShield from
http://muse.linuxmafia.org. Edit the config file, change appropriately, and set it to load at boot. Then point your internal LAN clients to it as the default gateway. Up and running.
Then, go learn about iptables (I'm still just use gSheild and I've been using Linux for a *long* time) for real, and maybe setup a transparent proxy, etc.