The only OS exam for MCDBA is Windows Server; no desktop exam is part of it.  The 2000 Pro or XP Pro could be used as an elective, but it's not a core requirement.

While there isn't a separate OS requirement for MCITP: DBA, the exams really pile on top of the one Windows Server exam as part of the SQL exams, including networking, ISA Server, clustering, etc.  If you feel you can pass 70-228 and 70-229 in the next six months, and you are not primarily working with SQL Server 2005 right now, then MCDBA is a good warm up to MCITP.  There is an upgrade path.  Once you complete MCDBA, you can take one upgrade exam (70-447)instead of the two MCITP: DBA exams.  You still need to take the MCTS exam, but that's pretty tame after getting MCDBA and spending a little time with SQL 2005 BOL.

However, if you are not already well on your way to SQL Server 2000 MCDBA, then you should consider spending your learning efforts and money on the current version.

-Eddie