Upgrade domain to 2003

In my opinion, every DC should be a DNS server.  You should use AD integrated zones (which are default).  There is no need to install the DNS service on the Windows 2003 server before hand.

I would keep the Windows 2000 machines running DNS before the migration.  Then add your Windows 2003 server to the domain.  run the domain and forestprep.  RUN DCPROMO on the Windows 2003 machine.  Then install DNS...and wait for the DNS database to replicate to the Windows 2003 server.  Transfer the FSMO roles to this Windows 2003 server.  Then make the new DC a Global Catalog.  Then remove the GC roles from the windows 2000 DC's.

Make your other Windows 2003 machine a DC by running DCPROMO.. after the installation, install DNS.  Make this server a GC too.  Point your clients and other servers to these Windows 2003 DNS servers.

As far as DNS configuration on the DNS servers themselves, follow these guidelines:  http://support.microsoft.com/default.aspx?scid=kb;en-us;825036

After run DCPROMO on the Windows 2000 machines to remove AD from them...

Before upgrading you should read through some of these links (skim each one initially).  

Here are some links that should get you going for upgrading a 2000 domain to 2003:

Common Mistakes When Upgrading a Windows 2000 Domain To a Windows 2003 Domain
http://support.microsoft.com/default.aspx?scid=kb;en-us;555040

Hotfixes to install before you run adprep /Forestprep on a Windows 2000 domain controller to prepare the Forest and domains for the addition of Windows Server 2003-based domain controllers
http://support.microsoft.com/?kbid=331161

Commodore.ca | Windows | How To Upgrade Windows 2000 Domain to Windows 2003 Server
Quote from the top of this article: "Several glossy Microsoft presenters have stated that all you need to do to complete a Windows 2003 Domain upgrade is run ADPREP and then upgrade away.  This may work for very small / simple environments but it is definitely not good advice for most companies.  After upgrading five servers in two unrelated domains and installing many fresh copies of 2003 I can say that I personally would not skip a single step in the process I have developed below."
http://www.commodore.ca/windows/windows_2003_upgrade.htm

How can I transfer some or all of the FSMO Roles from one DC to another?
http://www.petri.co.il/transferring_fsmo_roles.htm

How To Create or Move a Global Catalog in Windows 2000
http://support.microsoft.com/?kbid=313994

[If you run Exchange 2000] Windows Server 2003 adprep /forestprep Command Causes Mangled Attributes in Windows 2000 Forests That Contain Exchange 2000 Servers
http://support.microsoft.com/default.aspx?kbid=314649

Windows Server 2003 Upgrade Assistance Center
http://www.microsoft.com/windowsserver2003/upgrading/nt4/upgradeassistance/default.mspx

[If using R2 release of Windows 2003] Extending Your Active Directory Schema for New Features in Windows Server 2003 R2
http://www.microsoft.com/downloads/details.aspx?familyid=5B73CF03-84DD-480F-98F9-526EC09E9BA8&displaylang=en

Personally, I set the DNS settings on each DNS server to point to itself and itself only. With an AD integrated DNS zone, you shouldn't need to do any more.

Thanks for preciouse information..
One year ago in this domain windows 2000 I have upgrade Exchange form 5.5 to Exchange 2003 and ISA from 2000 to 2004. I had upgrade all my DC to SP4 sndextended schema for Exchange 2003 run adprep (forestprep/domainprep) from the CD of Exchange 2003.
Now, If I extend the schema for W2003 or W2003 R2, are there any problems for my Exchange 2003 Server and ISA 2004?

For you, which is the best and less risk technical for do this work?
Is to upgrade the domain or install a parallel domain and use ADMT tools?
In the second case how can I move my Exchange 2003 that is a member server of w2000 domain from old to the new windows 2003 domain without reinstall exchange?

An other questions, I don't have the password for restore active directory, how can I find it?
Thanks
MAurizio

thanks NJComputerNetworks,
i had just upgrade Exchange 2003. It is in windows 2000 domain enviroment.
I don't know if are there any problems for my exchange 2003 server after I upgrade the schema to prepare my windows 2000 domain to windows 2003 domain?