To benefit to the max bandwith of PCIE you SLI on x16/x8 , x8/x8 , the more expensive motherboards have x16/x16 , but the diference is 1% so is insignificant.
Yeah, I figured at x16/x4 would present a bottleneck, but I'm not sure just how much of an impact that would have on the bottom line performance-wise.
In any event, I ended up going with the ASRock Z68 Extreme4. On paper it has every feature I wanted at a fairly moderate price (under $150 with all the newegg discounts factored in, and with no messing about with rebates). I'm a bit worried about quality of some of the components and issues that have been reported with the iGPU, but I'll be using a discrete graphics card, so as long as that problem doesn't affect transcoding I think I'll be happy.
Thanks for the responses.