Check on the Quintum products (www.quintum.com). They were designed just for your situation. No need to replace your pbx or install cards, you just install the Quintum gateways onto your existing telco lines (between the pbx and the telco) and the quintum will intelligently route calls over IP as needed. Additionally, it will also allow for fail-over back to the telco if needed.
Look at their web site and feel free to contact me if you have any specific questions.