mitigation is the watchword
I don't want to worry you, but an upgrade from NT4 to server 2003 is very unlikely to go smoothly. My strongest advice is not to attempt any form of upgrade on the existing hardware - invest in a new server, preferably (more likely inevitably) with 2003 installed and working.
Then set up your users, access rights, backup strategy, utilities etc, etc and let them all bed down. Install your application with test data and similarly ensure it works as required.
Only then would I migrate the live data, keeping the old server warmed up for an immediate re-connection in the event of unexpected problems. To migrate your data hopefully, that would be a case of connecting old and new servers (with different IP addresses) and doing a simple copy. Actually, planning and testing the data migration is a problem of it's own that might take longer than expected.
Another point to consider is that of back-up. Hopefully, you already have a robust backup strategy, including nightly off-site storage of the media. I would point out it might be impossible to read your backup media (especially if tape) on a new server. I have never been able to do this. For one thing, you possibly cannot run your existing backup software on a new server, and have to upgrade that too.
Last upgrade I successfully retained my previous server as a warm-standby machine. The two servers are permanently connected, and every night a backup of all changed data occurs (I use SyncBackSE software - superb and very cheap). Every morning, therefore, the two servers are identical in content, and could be swapped in a matter of minutes.
I would allow at least 3-4 days of installation and parallel testing, and switch over would then be a couple of hours.
Good luck, and do try to let us know how it went.
