The difference is that 890GX comes with the SB850 southbridge which has native SATA3. The 790FX chipset is geared for people who intend to Crossfire heavy cards. As with the FX series boards you get the full dual x16 in your PCIe. With 890GX going dual means an x8/x8 crossfire only.
The 790GX chipset motherboard's feature onboard video.. The 890FX is current the top of the line chipset for AMD boards.. It does not feature any onboard video though.. It features full scale (16X + 16X) support for atleast two video cards in Crossfire and has better overclocking potential and features.. Also, its a native AM3 chipset supporting DDR3 RAM only whereas the 790GX is an AM2+ (AM3 ready) chipset with boards supporting DDR2 RAM as well..