Can I install an SQL 2005 SP 4 on a Windows Server 2008 Cluster?
Min SP3 required on Windows Server 2008 R2.
http://support.microsoft.com/kb/955725
Is there a way to just move the instances over or backup and restore them?
You can't move the instances. You can move the DBs and server-level objects.
The instance name needs to stay the name because we several application configured by the VirtualServer\instance name.
For retaining the SQL Name, You can either use Client Alias at the client level or DNS alias at the network level.
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SQLBuddy