Hi Kevin,
Here is some info to get you started. You should also read up on the Cisco Blueprint for success. There are many factors that will have to go into making this decision.
Infrastructure,staff,costs,support,migration paths, strategies etc. etc .etc. Not an easy decision, but done properly IP Telephony is a great way to go. Done improperly, it can be a VERY painful experience. Have a look;
IP Communications (VoIP)
Case Studies
http://www.cisco.com/en/US/netsol/ns641/networking_solutions_customer_success_stories_list.html
IP Communications (VoIP)
Introduction
http://www.cisco.com/en/US/netsol/ns641/networking_solutions_packages_list.html
Hope this helps!
Rob