Building a Secure Linux Gateway

My machine is an old Dell Optiplex 466/MXe, it is a 486 DX2 66, with 16 MB of RAM, a 512 MB Hard Drive, a sound card and a 4X IDE CDROM. I acquired this one for $50 and upgraded it to a 486DX4 100, 40 MB of RAM, I removed the sound card and added 2 network cards, a SCSI card and installed a 320 MB SCSI hard drive, all of which I had in spare parts. The minimum system you will need, is a 486 (any flavor), 16 MB of RAM, 200 MB hard drive, two network cards and either a CDROM or the ability to do a network install. You will also need a copy of RedHat Linux 6.x. Although any distribution will work just fine, I will only cover RedHat. The system will only need a monitor during the install, after that it can run headless and can be administered remotely using Openssh.