I'm looking mATX motherboard & case that supports 2 dual slot GPU's with 1 slot in between. So the perfect PCIe layout would be something like:
x16 GPU 1
x1 overlapped
x4 overlapped
x8 GPU2
I have checked Gigabyte, MSI, ASRock and ASUS but couldn't find anything in the Z270 range. The only mobo I found is the that could fit the GPUs would be the Gigabyte Z270MX-G5 and has the following layout: x16 x1 x8 x4. But I don't think its a good idea to use the x4 for a second SLI GPU (but I don't really know).
IIRC Nvidia requires at least x8 for sli (not that there's really a significant difference in gaming performance between x4 and x8 or even x16... nvidia has just decided to lock it to x8).
The older article on pugetsystems links to AnandTech and TechPowerup measurements that indicate a small performance loss with x4 (tests aren't doing SLI though, just single card).
The following is a quote from the Gigabyte mobo manual:
Observe the steps in "1-5 Installing an Expansion Card" and install CrossFire/SLI graphics cards on the PCI Express x16 slots. (To set up a 2-Way configuration, we recommend installing the graphics cards on the PCIEX16 and PCIEX8 slots.)
I've seen articles where they (duck)tape some PCIe pins to force the GPUs to run in x4 - but still using the x16 or x8 slots. Is there some verification/certification going on?
EDIT: I think its a certification thing but I'm not sure if its pinned to specific slots.
