I know its an old thread, but I have to note that according to the specifications the PCI-E slots on those mobos are set up like this:
990fx - x16,x16,x4 (dual x16 for Crossfire/SLI)
990x - x16,x8,x4 (x8 x8 for Crossfire/SLI)
970 - x16,x4 (x16 x4 for Crossfire/SLI)
Not 100% sure about the 3 way Crossfire/SLI, but I think its something like this:
990fx - x16 x8 x8
990x - x8 x8 x4
970 - N/A
-- Personally I would suggest the 990FX as well. You can geta really nice BioStar 990FX mobo. Thats only ~$30 more than the 970 series mobos, and you get so so much more.