The Cumulative update package 12 for SQL Server 2005 Service Pack 3 (Build 9.00.4311) is available at:

http://support.microsoft.com/kb/2345449

And here you can get the whole information of builds that were released after SQL Server 2005 builds that were released after SQL Server 2005 Service Pack 3 was released: http://support.microsoft.com/kb/960598/

CU
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