Ok, a few questions for you first:
1) Is the old NT box a domain controller?
If so, are you using raoming profiles? If not the answer for user accounts is simple. Start your new server as a BDC and synchronize it with the PDC, once done take the old NT box down and promote the new NT box to PDC, now it's in and the user accounts are there and ready to go, seemlessly.
2) The "DATA" you are refering to, is that mapped network drives? If so that makes things a bit more difficult, you will most likly have to do this swap during off-peak, or overtime hours. First off, if it is mapped drives, you will need to recreate the directory structure on the server EXACTLY as it was on the old server, secondly, if you use a logon script to map these drives, you will need to change it. HOWEVER, if you set up the structre the same as the old box, you will only need to change the name of the server in the logon script. A way to avoid this in the future it to use the IP address of the server in the logon script so that once the old server is removed, you can change the new server to the old servers IP address and things should then work seemlessly with out changing the logon script.
If there are more questions, or you have a different set-up, post more details, and I will give you another scenario to use to complete your task as easily as possible.
Good Luck,
SteelDragon