Exchange via WAN links ?

We have a variety of small sites with no formal I.T. staff but between 10 and 40 users. Planned to outsource the basic support(desktop, printer, F&P) services but needed to keep Exchange and Domain Controllers under our corporate control back at USAcorporate office. Several of these sites are international w/users connecting back to corporate over 128K WAN links with avg 200ms response times on ping tests. Problem is Exchange is very slow. Bandwidth utilization is averaging less than 50%. Doesanyone know where I might find info on Exchange latency requirements?
I welcome any advice on what you would do to serve these offices as our current model seems to be doomed. Thanks much.
Gary