Mirroring a folder over a WAN

I have a number of users in the UK who need to access large files stored on a US server. The offices are linked by a T1 (1.554Mb) pipe and I would like to ensure:

1. People from both sites can access the files quickly.
2. Any synchronization of files ensures that no data is overwritten (ie. no chance of 2 users making changes to the same file)

Platform is NT 4.0 Server, clients 2000 Professional, NT 4.0 Workstation & 98 SE2. Network protocols: TCP/IP only.