I guess the ultimate in setting up a small PBX is Asterisk, and one day soon I'll be investigating that further. In the meantime, I have converted one of my landlines to VOIP using my cable internet. I have to keep one physical line for emergencies, faxing ( faxing over voip is not always reliable ) and eftpos. I have purchased a Linksys SPA-2102 that does the VOIP. I just plug my old telephone into it. The box does the rest. The neat thing is that my voice messages are cc'd to me by email. As I understand it, once I get an Asterisk server setup, I can ditch the SPA2102, and connect the cable directly. I will need FXO cards though to do the translation.
I'll be researching phone systems - both the System and the Phones. For one secretary I want her to be able to walk around the office to accomplish things, but also to be able to answer phones easily. I think Asterisk would be awesome.
I don't see Asterisk on the cards for another couple of years. It's too high tech for most users and so will appeal to only a very small group of users.
If not Asterisk, there must be some Windows based software that would allow some telephony stuff. like http://www.mycallsoft.com/
When you get it working with your office, let me know and I'll have a look at it. That link to call soft is just voice/fax mail software.
I know of a guy using this service. I have heard they will setup the database to integrate with your existing software.