If your going with a Cisco System (CM or CME), you might want to consider SIP trunks from a VoIP provider. They are inexpensive and can install directly to the Cisco system with no other equipment needed. All you need is a decent Internet connection (like a T1) and you'll need to open ports on your firewall. They work great and you can keep your numbers too.

I have more info on a great SIP trunk provider if you need it.