Setup VoIP within our network?

We are looking into setting up VoIP within our network. We are not really sure how to go about this. We don't want to use a carrier and have thought about using Cisco's VoIP Infrastructure Solutions for SIP. Would we still need to use our PBX, or could we hybrid PBX and VoIP? How would we go about this and what are the advantages/disadvantages of it over digital? Does anyone know where I can find information on this as well?