When you build the 2 new Windows 2000/03 servers it would be easier if you load DNS on the servers. This will allow you to move the DNS zone for the Windows 2000 domain into an Active Directory Integrated zone. AD integrated zones are easier to manage than traditional Primary DNS zones like the one that exists on your current DNS server.
In order do this you could:
1) Install a new server
2) Make it a secondary server for the DNS zone
3) Allow a Zone transfer to occur
4) Make the new server the primary DNS for the zone
5) Use DCPROMO to make the new server the first domain controller in the Windows Active Directory Domain
6) Build the 2nd new server (Installed DNS on the server, but make sure it points to new server 1 as the primary DNS).
7) Use DCPROMO to make the 2nd new server another domain controller
Now all systems can use New Server 1 as the primary DNS and New Server 2 as the secondary DNS. The DNS on the old WIndows 2000 server can be uninstalled if you like. Keep in mind that only YOU have all the information about the configuration of the DNS server and the DNS zones. My response assumes that the only zone on the old DNS server is the zone that represents your internal corporate infrastructure, which you will be using as the AD domain. If you have external systems pointing to the current Windows 2000 DNS server, or additional zones on the server you will need to take appropriate steps.
As for the DHCP server, you will want to join the AD domain as soon as it is built. You will then need to AUTHORIZE the DHCP server to act on systems within the domain. This is done in the DHCP MMC by right clicking on the server and clicking Authorize. If you do not perform this step, your DHCP server will not give out IP addresses.
Please keep in mind that these are general steps. There is no replacment for proper planning and testing.
If you have further questions, feel free to post them.