We have an NT domain and we would like to upgrade to Windows 2003 server. Our current setup is below:

1 NT PDC
2 NT BDC
250 Client Computers (laptop and desktop)

All of our servers meet the requirements for Windows 2003 Server. We use our servers for login authentication only. We are using samba for file sharing and postfix for email.

What is the best option for upgrade? Can anyone please point me to the right direction? I am new to this process and I would appreciate a very detailed instruction> Thanks for all the help.

Upgrade your NT PDC 1st (be sure to have a full backup, that your sure is valid)
also, useraccounts are migrated, but passwords are not, so you must reset all passwords to the AD domain.

if your using *nix for filesharing/email, i dont believe you really need to do anything special, you can upgrade to the latest version of samba which has a hook in it ot integrate with active directory

Please tell me you are going to d a clean install and not doing a literal server os 'upgrade'? If you just do an upgrade, the nw will never run as well as it could. Also, you menaiton that you are new to this? If you are not familiar with it, you may want to bring in a consultant as it sounds like you may be in over your head. You have 250 users that have to use this network so it is going to be a little more involved than setting up a home peer to peer.