Best way to migrate is to install the new 2003 server into the same org as the 5.5 server, install the ADC and then move the mailboxes accross using the mailbox move wizard in Users & Computers. This is the official MS way. I've done a number of large migrations with this strategy. It has minimal impact on the end users etc etc.
It is NOT the most straight forward way though. Exmerge would also work BUT you loose the single instance store (i saw a Exchange store go from 30gb to 70gb from this method) and the mailboxes you Exmerge back up wil loose the ability to reply to mails received prior to the migration since you've in effect deleted the mailboxes.
Seriously mate this is complicated stuff, every strategy has pros and cons and i've been doing Exchange migrations and upgrades since version 4.0.
Don't do it, you need help. If you can't afford to get professional help on this then you can't afford to do it full stop.
Your boss needs a reality check before he gets his arse burnt
