Microsoft has released Cumulative Update 2 for SQL Server 2005 Service Pack 3, which you can request here. It is Build 4211. The fix list for this CU is fairly short.
My recommendation for people that might still be running old builds of SQL Server 2005 is to install SP3 and then install CU2 for SP3 rather than to install CU12 on top of SQL Server 2005 SP2. I would follow the same advice if I were installing a fresh copy of SQL Server 2005 RTM.