Let it be clear that AMD chipsets do not support nVidia SLI.
The best chipset for dual CrossFire would be the 790FX, which can have up to four PCIe x16 slots. It supports the following CrossFire modes: 16x16, 16x8x8, and 8x8x8x8 via it's PCIe pipeline.
After that, either the 790X or 790GX with two PCIe x16 slots. Such boards support single PCIe x16 or 8x8 CrossFire. Keep in mind though, that not all 790GX motherboards use a 2nd x16 PCIe slot. Some are using x4 slots, thus limiting CrossFire mode to 8x4.
The 785G boards use a single x16 slot and an x4 slot. This would only support 8x4 CrossFire mode.
The 890GX is brand new, and since it's in it's infancy, it will go through some teething pains. I suspect every motherboard using it will have regular BIOS revisions for several months.