I must admit that I don't have much experience with large networks. But I'll try to give you some ideas to work on for centralised applications. All your clients will be connected to main office either with T1(or other solution) or with RAS server(that is most of the time modem connection on client side). You could consider two tipes of app server.
1. Linux/Unix app server with telnet/ssh access (I'm not sure if you could have GUI)
2. W2K app server with terminal services (GUI will work... tested)

In both cases 33.6 kbs per user will do, because all processing is done on server. Secon solution would probably ask for more hardware power (cluster???) but ofers classic MS GUI that most users are used to work with.

I hope I was helpfull
for more info about MS solution search knowledge base at support.microsoft.com