You can get an Intel Johnstown board (Atom N270 - 1.6ghz) for around $110. RAM will set you back another $20 or so. The Johnstown board is nice in that it has a built-in PSU and just requires an external 12v brick. If you don't have one, that's another $20. For the chassis, it depends on what sort of storage you want to put in it. Anything that'll hold an ITX board will do, though I like the mini-box cases (an M350 will set you back $40 though it requires the use of laptop drives). An M300 is more expensive at $60 but allows the use of a single 3.5" drive so you can recycle a hard drive you already own.
Anyway, I think the best bang for the buck is one of the Intel Mini-ITX boards. They're inexpensive, capable and familiar. Be aware that the older Intel boards (Little Falls, Little Falls 2) have no integrated PSUs so that drives their prices up relative to the Johnstown.
Like, ew and stuff.
Once again, XKCD predicts the future:
Dang that guy is scary