Looking for Advice to tackle a migration

Current Situation:

Client has many POP3 mail accounts...Everyone's Inboxs becomes due to limited space @ ISP....they currently are running 4 NT servers and one 2003 memeber server with SQL

I would like to upgrade them to 2003 domain structure and install exchange 2003...

I haven't attempted something like this before looking for some do's and don'ts

Do you guys have any advice to tackle this migration?