This is an easily done upgrade. It’s basically just as complex as a normal SQL 2000 to SQL 2005 upgrade.
Make sure that all your code is SQL 2005 compliant.
Stop SQL on the SQL 2000 server.
Copy the data files from the SQL 2000 server to the SQL 2005 server.
Attach the files to the SQL 2005 server.
Rename the SQL 2000 server
Rename the SQL 2005 server to match the name of the SQL 2000 server.
sp_dropserver / sp_addserver
Add any jobs, packages etc which need to be added to the server.
Update database statistics for all tables.
Smoke test the system.
Allow the clients to connect again.
The server rename part is optional, but that way you don’t have to change the connection strings on all the client machines.