I have been told that we can greatly reduce our processing times by rebuilding our 2000 cubes in 2008. I was told the difference is substantial (2hrs down to 20mins).
Is this true? If so, why?
Performance usually improves from version to version of SQL Server. SQL Server 2000's Analysis Services was nowhere near quick.
As to why - Microsoft doesn't give us the source code so it's a little tough to analyze that. ;-)
Like @ConcernedOfTunbridgeWells pointed out in his comment, though, the real reason you should get off SQL Server 2000 is that it's out of support.