Migrating from NT4 to What???

Id stick to 2000. 2K3 is too new, although it might offer a lot. Are you going to replace all desktops too? Do you have any non-pc items that sit on your network? ie) a NAS box , scanners, etc?

Some of the older stuff might not work with 2K3, but should work with 2000. Of course you never know till you check it out.

I just went to an MS presentation on that very subject. In fact they have a website just for that. I have a similar setup and am facing the same problem. MS realized that a lot of people are in a similar mode and created some impressive tools to make the leap from NT to 2003. A bigger question would be why do that? It would be much better to start a fresh install on a new server of any OS then to migrate several versions. Unless you have a specific need for 2003, and only want to upgrade (not start fresh) then I would go with upgrading to 2K. Note that Exchange 5.5 and 2000 will not run on a 2003 server.

I would suggest W2003 over w2k. first check if your existing apps work in w2003 which currently work in NT4. If you have any 16bit apps, then you may have trouble. Decide if you are going to use 32bit or 64 bit of w2003. you will have mailserver in w2003 which has one more reason for you. you will have much better DNS/AD support in w2003 than in w2k. If you have AD experience in w2k you would immediately say yes to w2003.
Any one day you have to move to 64 bit W2003. why move to w2k in betweenand face active directory complexities. do once with w2003 and forget it.

RV

most OS upgrades require hefty hardware upgrades or replacement. You may want to take this opertunity to evaluate Linux, at least for the back office if not the desktop. Win2003 imposes requirements that would dictate wholesale replacement of hardware and upgrade or replacement of much software. Will your verticle apps run on the next desktop OS, do they require an MS server platform, do they need Exchange? The price of a ticket to ride MS's merrygoround increases geometricly. Win2003 isn't even released yet, forget the stability/security issues, it's time to explore options.