Think small. This new Mac Mini rocks! It is tiny which is what anyone at home or in a small office needs. With 1.25 or 1.42 mhz G4 processor and 40 or 80 mb RAM at $499 and $599, the affordability is to die for.
The Mini has the latest OS X Panther plus iLife '05 (iTunes 4.7, iPhoto 5, iMovie HD, iDVD 5 and GarageBand) on it. You can always steal the iLife stuff for one of your client machines because I'll want you to keep your Mini as a pure FileMaker server (even if it is just running the FileMaker client as opposed to the FileMaker Server software).
The $500 price leaves you with no excuse for trying to serve FileMaker files from one of your regular work machines. Keep FileMaker serving files with 100% reliability and keep those files in pristine condition!
Remember that it comes sans keyboard, mouse and screen. So you may want to buy a brand new mouse and keyboard for it and a tiny monitor - or use an existing monitor. You can get a 14" flat panel for something like $200 these days.