The short answer to your question is that Microsoft plans to support Win2k and XP/Server .Net exams in parallel, so you can mix and match them to your heart's content.
There's no compelling reason to wait, nor any compelling reason to focus only on Windows 2000 (except perhaps, for the very real consideration that many organizations wait a year or longer to deploy new operating systems across the board). But no, you can go ahead and start now. You don't need to wait for the new exams to become available.
For Microsoft's official statement on these subjects, please visit: http://www.microsoft.com/trainingandservices/default.asp?PageID=mcp&PageCall =faq&SubSite=cert/mcse&AnnMenu=mcse#3title
The title of this portion of the MCSE FAQ is "Frequently Asked Questions about Windows XP Professional and Windows .Net Server Exams" so you know it speaks directly to your concerns.
Good luck on your upcoming certifications!
This was first published in September 2001