Does anyone know more details about "downgrade rights". Do you need to ask
MS for this right? How do you get a CD key (a legit key) for SQL 2000 if
you urchase SQL 2005.

From MS Licensing FAQ at: http://www.microsoft.com/sql/howtobuy/faq.mspx
Q.
How do I downgrade to SQL Server 2000?




A.
If you require SQL Server 2000 for existing solutions, you can still
acquire additional licenses by taking advantage of the SQL Server 2005
"downgrade rights." The downgrade rights enable you to purchase server plus
device CALs, server plus user CALs, or processor licenses for SQL Server
2005 and install and use the previous version for a reasonable period of
time.