IT consolidation with Geographically dispersed offices

I have been tasked with consolidation the IT Infrastructure for a company that has 4 geographically dipersed locations.

I am charged with e-mail consolidation, TS access to remote servers, Remote printing, remote file access.

Currently, email is handled by:
Office 1: Hosted Pop3
Office 2: Exchange (not sure wich version)
Office 3: Exchange (not sure wich version)
Office 4: No e-mail yet
Roming Users: Mostly yahoo and gmail.

I can't decide if I want a Geo Dispersed Exchange cluster, or have one exchange server and have Outlook use one server with OST files.

Each mail system above uses a different domain, we want to consoldate all e-mail to a new domain.

Please ask questions, as I am sure I have not given enough info.