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.
790FX based boards (the newer ones with the SB750 not the SB600), are what you want to use if you are using two high end cards. For a more midrange solution a cheaper board based on the 790X, 790GX, or 890GX may be better for you simply from a cost/performance perspective. If you want to use SLI you only have two choices for the chipset, the 750a and the 780a (renamed the 980). Of course, you should go with AM3 for a new system so if you want SLI then the only two solutions come from MSI, and thankfully that are pretty decent boards. Just remember that nVidia drivers prevent you from running crossfire on those boards.