Upgrading from 2000 is a much bigger deal than from 2005, just because of the differences in the versions.
Before you upgrade the 2000 server, run the following:
DBCC CheckDB
DBCC CheckCatalog
If there are any errors at all, do not proceed with the upgrade until you've fixed the errors.
Once you've upgraded, run DBCC CheckDB WITH Data_Purity and make sure it comes back clean. Run DBCC UpdateUsage and update all statistics with fullscan.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability