For something as misison-critical as a departmental server, I recommend against attempting a "migration" upgrade. I always do new servers from scratch. That's my standard procedure for all operating systems, not just NT4->NT5, I don't trust any of the migration wizards to do it right.

Besides, you should never miss an opportunity to clean an OS completely and start from scratch. Systems always run faster and more stably after a re-install of the OS because you don't have all the old unused crap such as old drivers and old software modules.

Your only trick will be if you're using domain security and that NT4 server is the domain controller. If so, you'll need to do the Domain Controller Dance (tm) as part of the migration. I've done this before, following Microsoft's instructions, and it's not that difficult, it's just time-consuming.