There is no direct upgrade path for 2000. You have to upgrade from Windows 2000 to 2003 MSCE by taking 2 upgrade tests, and then you upgrade to Windows 2008 with at least 2 more upgrade tests. Or you can start fresh with a 2008 cert.
From MCSE 2000 to MCITP: Enterprise Administrator, you would need to take the 70-292, 70-296, 70-649, 70-647, and the 70-620 or 70-624. So there are 5 exams total you would need.
If you only want to earn MCITP: Server Administrator you would need the 70-292, 70-296, 70-649, and 70-646. This lowers the exam count to 4.
http://www.microsoft.com/learning/en/us/certification/mcitp.aspx#
