Asterisk looks like it's pretty cool, it's an open source PBX phone system, and my obsession this week. It looks like it can save you a lot of money and give you a lot of features.
I've been trying to figure out how and if I can get faxes through it here.
Someone else, named Scott, tried it and wrote about it here.
Also he writes a lot about Asterisk and IP phones here.
This guy explains why faxing over a VoIP can be such a pain. Basically VoIP is compressed into a stream smaller than a FAX is compressed into, and will cancel out some FAX noises.
Equipment
Six port FXO PCI card: A40003
Place to buy VoIP equipment. VoIPSupply.com
4 FXO port Rhino card.
I think I'll recommend the Digium TDM04B 4 port FXO for us. It's under $400
We'll probably need a PoE switch now. This switch is managed, I've never used a managed switch before. Linksys