Difficult to say without knowing how much traffic you're going to be pumping through, and how many branches you're eventually going to have.
If traffic will be minimal or even average, you could get away with 1700's (with VPN modules) at each branch and say, a 2600/3600 (again with VPN modules) at the central office.
If traffic is going to be huge, probably go with something like 2600/3600's at the branch and a 7200VXR at the central site.
You'd be best to call your local Cisco sales office and have an SE come and visit and scope out exactly what you're trying to do, they'll come up with a recommendation for you.