Sorry about the late reply, just saw this thread now.
with about 25 active users, up to 50 at max, mostly file sharing and back-ups, all users run client machines with off-line capability
That can all easily be handled by a dual-core.
If data access is what it's mainly used for then investing in faster &/or more storage solution plus hardware GbE ports would be a better upgrade.
Server 2008 uses SMB2 for file sharing which is faster than SMB, what Server 2003 uses. In order to take advantage of SMB2 you have to take into consideration all clients must run Vista as well.
[EDITED]You can read the exact details about SMB2 here