When the BIOS first came out in April, the 4C was for Quads and the 2C for dual cores. In the 521 release, they changed the BIOS configuration but did not change the nomenclature making it very difficult for people with a Q6600 to know what to use. This was confirmed by Raja on the DFI forums.
Now the beta bios releases that have been posted lately appear to be unified but I haven't tried it.
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