In this case, I would guess: Single machine!
For low traffic, only a few connections via webdirect, 'single machine' is OK. For heavy traffic (wd etc.), one machine can host the database server, another machine the web-part (web publishing engine, web-server,)
(the server manual has a very good section about installing-scenarios, btw)
First, do you have two machines you're prepared to dedicate to FMS?