I was asked on Monday about issues concerning installing SQL 2000 for the Reporting Service database and using SQL 2005 services.  There is a whitepaper covering multiple installations.  You need to be careful as there are a few caveats to be aware of.
The SQL Server 2000 version must be the default instance and the SQL Server 2005 can be a named instance.
If you use two versions SQL 2000 can’t use SQL 2005 reports, but SQL 2005 will ask you to upgrade SQL 2000 reports.

Do read the white paper and make sure it will work the way you expect it to.