I did the multiple F@H clients on a network through NAT'ed dialup until getting DSL recently. That was before I knew about Timeless Tinkers (a pet name for work units using the Tinker core and without deadlines) with the v5 client, so the occasional large Gromacs could be a PITA. Anyway, you just need one computer set up to be the "server" with ICS if you're using Windows 2000 or XP. You could also use Windows 2003 Server with Routing and Remote Access Services (which I did for a short while), or a Linux system with demand-dial PPP (or whatever they call it).
Gleem has covered most everything you need to know about settings, but if you have trouble getting the F@H client that is installed on the dialup server itself to trigger an autodial, shoot me a PM and I'll try to help you through it as I've dealt with that issue multiple times. It's not fun when everyone on the official F@H support forum blows you off and ignores that the issue exists. (sorry, can you tell that I hate having to deal with autodial settings on Windows?) Oh, and the new v5 client has improved sending procedures to ensure that WUs get sent on connections that take a minute to get established. Us dialup old-timers who remember having to shut down the service and do a "-send all" every week or so to flush out the "stuck" WUs appreciate the new v5 clients a lot! 🙂
Sorry, I'm rambling, I'll shut up now...