The basic difference between 790FX and 790GX/790X is the amount of PCI-E lanes.
790FX allows you to use 2 x PCI-E 16X slots at 16X bandwidth
790X/790GX allow you to use one PCI-E 16 slot at 16X, or two 16X slots at 8X each
I don't think it really makes any difference when crossfiring current cards though.
But, the Asus M4A79 Deluxe is a very good board for overclocking.