What you propose (finish the MCSA on Win2000 and then upgrade to Win2003) is not only possible, but probably the fastest path for you to follow. You'll want to get moving on this quickly, though, because with Vista Server in the offing for 2007, Win2000's days (and related certs) are numbered. Get 70-216 out of the way right away, then start working on the 70-292 immediately thereafter. By the time the new programs come out for Vista, you'll be ready to take that upgrade as well and keep current with the ever-changing Microsoft cert landscape.
This is doable, not too expensive, and probably won't take more than two weekends a month for three months for 70-216 and twice that long for the 70-292 exam. You should be able to finish both before the end of this year, but you need to get moving now.
Best of luck, and thanks for posting.
This was first published in January 2006