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Dell or any OEM very rarely lists the CPU's that the BIOS (CPUID) will support; they want you to buy a new PC. Therefore, you will need to contact Dell directly -> http://support.dell.com/support/index.aspx?
ref - Dell OptiPlex GX60 - http://support.dell.com/support/edocs/systems/opgx60/en/ug/specs.htm#1112435
IMO - upgrading the CPU is a huge waste of money and speed increase will be very minimal in 'today's' world. The cheapest new PC at Best Buy will run circles around your GX 60 regardless of the CPU you can put in it, and have ALL new parts.