I have applied SP4 Build 5324 to a SQL Server 2005 Enterprise active/passive cluster (Build pathway: 4053 to 5000 to 5324).
The SQL Server log contains zero errors.
The SQL Server Agent log contains the following records continuously written since the Build was completed:
- [LOG] Unable to read local eventlog (reason: The event log file is corrupted)
- [LOG] Successfully re-opened the local eventlog - NOTE: Some events may have been missed
Email messages that are marked with dates days and months prior to applying SP4 Build 5324 were subsequently resent  hundreds of times [/edit] ("storm" would be an apt description).
[update] I've confirmed that there were zero unsent items (sysmail_faileditems and sysmail_unsentitems views) prior to applying the service packs. [/update]
If you have encountered this behavior, I would appreciate learning if you identified the cause.
PS: Yes, it is well past the time to upgrade to a newer version of SQL Server. Let's leave that discussion for another day. ;-)