Migration from Exchange 5.5 in NT4 domain

[T0002636]

Hi,

My problem is like this:
Number of users = 150 ~ 200
PDC = email server in NT4 domain
current email server = exchange 5.5
user PC OS = Window XP Professional

Objectives:
1. Split email server and PDC into 2 separate servers.
2. Migrate PDC from NT4 domain to Win2k / others ? I learnt Samba server can be used as PDC. But I don't know if Samba server = Win2k domain ???
3. Copy email accounts, emails and public folders from exchange 5.5 to Zimbra
in the SAME domain. My boss does not want to change domain name in user's PC.

I am totally new to email migration but this is my task ~_~
Would somebody help please ?