upgrade from NT4/Ex5.5 to MS2003/Ex 2003

I intend to upgrade my exchange 5.5 server running on WinNT 4.0 to Exchage server 2003 on Win 2003. At the moment i have a NT4 server which is the PDC and on the the 2 BDCs i installed the Exchage 5.5 server. Each exchange server has a differnt DNS sufix. Clients use Outlook express and outlook to access the two different mailboxes. example, ty@domainA.com and ty@domainB.com respectively. How will i upgrade. Very urgent