BEFORE you move off of SQL 2000 run these against the databases. If there are any errors do NOT backup/restore to SQL2008, fix the errors first.

DBCC CheckCatalog

go

DBCC CHECKALLOC

go

After you backup/restore to SQL2008 run the DBCC UPDATEUSAGE(0, N'dbo.tablenamehere') on ALL tables in the database.

Then rebuild ALL indexes on ALL tables so the optimizer will see all of the indexes in the SQL2008 optimizer. Once that is done run a DBCC CHECK DB on the database to make sure there is no corruption at all, then do a db backup so you have a good SQL2008 Recovery point before you open it up to testing and customers.