I was running a State sporting organisation on a Pentium 200, 64 RAM, 2.5G HDD with SME 5.5 for a number of months. The SME was doing everything (PDC, Web server, email server, file share, print share), with up to 9 Win9x clients, networked digital photocopier, laser printer. Recents use of awstats show the server was also getting a hiding from external users viewing the website.
Never had a complaint. Ironically, I recently replaced with a new Dell PowerEdge server, and had quite a few teething problems before getting it settled.
My current test server is only P166, 128 RAM, 3.2G HDD running 5.6. Although slow with server manager, it works just fine.