Just to be totally correct, there is no Windows XP server. Microsoft is due to release .NET Server sometime next year. The latest shipping server product in Windows 2000 Server.
Having said that:
1. I assume that the NT Server is your PDC.Do you have any BDCs. If so, make sure you take one off-line (disconnect from the network) before starting your migration to Win2K (and Active Directory) on the NT4 PDC. This off-line BDC will provide you with a fall-back position should your migration go wrong (this is a Microsoft product; Bad things sometimed happen).
2. Make sure all the computers meet minimum hardware recommendations before starting your upgrade. BTW, if you stick with the MINIMUM requirements, you will NOT be happywith how th computers perform. I would at least double what MS lists as minimum requirements. Microsoft has a compatibility checker available either on the XP CD or as a download. When you run this utility on your workstations, it will provide you a list of what hardware or installed software may not be compatible with WinXP. This is especially critical on the Win98 computers depending on how old they are. Since WinXP is essentially an expanded Windows 2000, you should have no compatibility problems there.
3. Both Win2K Server and WinXP come with Novell clients. Have not heard of any major problems. You might want to search the MS Knowledgebase to see what problems have been cropping up.
Hope this helps.
