Windows

you haven't talked about whether there is user data on the existing servers that are listed, which changes how things get done. i would pull the BDC with nothing on it off the network by unplugging it. (this is a safety measure in case somethign goes wrong you can revert back to your original network state) than upgrade the other bdc with dns dhcp on it to windows 2000 than demote the pdc and promote the dns/dhcp member server to be a domain controller. now redo or upgrade the old pdc to 2000 and if everythingis working redo the bdc you pulled off the network.

There is no user data in the PDC or the DNS/DHCP.

I thought that DNS and DHCP would need to be split up before upgrading?

I have to agree with Kosmo... that would be the safest and most efficient route to upgrade your servers. And to answer your other question, No- DNS and DHCP can be run on the same server. In fact, you could run DNS, DHCP, and WINS on the same server. However, I would not recommend this due to the overhead.

I would complete the upgrade as Kosmo stated, then I would transfer the DHCP to the final server you upgrade (the BDC that was unplugged for disaster recovery purposes)after it has been confirmed that all your services are running smoothly.

This will help to spread out some of your server overhead, since both DNS and DHCP can be demanding on a server, depending on the number of name resolution queries it deals with and the durration of any DHCP leases.

Start reading here:
http://tinyurl.com/2y48h

I am pretty sure i have read that the "MS" way is to pull the PDC offline, and update a BDC to the domain controller with win2k. THat way, if the network gets screwed up, you still have a working PDC to fall back on...you do not talk about your clients...I am assuming they are win9x or NT since you are running WINS, and specify that is the problem. If that is the case, I would setup wins on the BDC running DNS and such, pull the PDC, and then upgrade that BDC to Win2k...

in addition to the contribution above. You could use the ADMT(Active Directory Migration Tools) to migrate your users and exchange mailboxes if you have one. And you could keep all the ACLs on the folders and files that your users have. Have a look.
http://www.microsoft.com/downloads/details.aspx?FamilyID=788975b1-5849-4707-9817-8c9773c25c6c&DisplayLang=en