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im thinking of going with a I7-3820 on a x79 mobo but would I be better going for a Z77 mobo and either the I5-3570K or the I7-3770K cpus's?
Get an Intel for a start. After that something like 3570K would be a good choice.
The AMD Piledriver 8350 is a very good option as the op already has a compatible motherboard.
Im thinking of going Matx for my mobo