If you have quad channel then 4x2gB (lga 2011, X79 chipset)
If you have dual channel 2x4gB.
Four 2gB cards will slightly be faster than 2 4gB cards in any config. But they will fill up the slots also you don't get a noticeable boost. So it is better to go for two 4gB cards or even if you can afford and will be needing more memory in future four 4gB ram cards.