VoIP Design

I am rookie in voip. I have two sites in the same city linked by fibre and on voip using quintum voice gateways. One is using 2611XM and the other 1721 routers. I need to upgrade the network to include two other sites still in the same city and one more in a different city 420km away (we are linked by a vpn tunnel). I also need to use Cisco equipment. I was thinking in terms of 3800 for the headoffice, 2800/1800 for branches. I am not sure what else is required. Can anyone out there help?