if your planning to upgrade to vCenter Server 5.0 Update 1a and use Oracle as the backend database you may want to check out the following KB before you proceed.

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2032277

There is a known issue that will show error: PLS-00905 STATS-ROLLUP2_PROC as invalid and prevent the upgrade from completing leaving you in an unusable state. Full resolution and recovery steps are described in the KB article