BleepingComputer.com: Finding the right server

For the web and secure servers, WebNPO wants 3 Intel-based machines each with three ethernet interfaces: One will be rigged as the firewall/router/proxy, another will be the secure app & web server, and the third will be rsynched with the app server and serve as a warm backup server. See the attached sketch for the rack layout and jumpering. These do not have to be super-powerful servers, single-socket dual or quad core Pentium or AMD will work, 8 Gigs of RAM for each. This way any of the three can serve in any capacity.

^ I need help finding the right hardware, I'm not exactly sure what I can use. Please help thanks! I was checking out the HP ProLiant, but wasn't too sure.