What a lot of people do not realize though, is that Stealth wants to make his Mac's join a DOMAIN on a Windows Server 2003 machine. Networking with Windows XP and Windows Server 2003 is VERY DIFFERENT! Windows XP users can simply get by with workgroups, and there's where a Mac will join up in a network using SMB. But, when running a domain with a SERVER-Based OS, it's a LOT more complicated.
If you have your server running Windows XP Pro, XP machines do NOT use Active Directory for server/client communications. It's the Server OS that runs Active Directory, and THAT is the key here. When the Mac's try to talk to the domain controller, they have to go through Microsoft's Active Directory, or whatever other database you may have set up, such as LDAP. The problem here is that using something OTHER than Active Directory can cause more headaches than necessary. Getting something that is compatible with Active Directory, and according to one of the links I have posted, getting either Panther (10.3) or Tiger (10.4) will directly communicate with Active Directory, Authenticate, and look just like a windows client on the server.
Now, Kamikaze is correct, but what you're describing there is having his mac communicate with an XP Based server. SMB works with workgroups, not Active Directory, and you cannot just use Workgroups with Windows Server 2K3.
The mac OS X 10.2 and below will only authenticate with LDAP. LDAP is not a built-in piece of software that 2003 uses. The only reason you would want to use LDAP is if your server is a Unix/Linux based server, or you're running the dreaded Novell Netware 6 (in my experiences anyway)
If you're planning on upgrading, you might as well just upgrade to 10.4 - no use upgrading to 10.3 and having to pay for 10.4 . IMHO, Tiger is a service pack to Panther, but I dont delve into the Mac side of computers near as deep as I do in Windows.
( four edits - it's time for bed. )