ive got a 6x 750ti rig running right now using this. gigabyte windforce gpus on a z87-g45 mobo... trying to get a 7th gpu to recognize.
I took have (a couple) of z87-g65 motherboards filled up with Windforce 750 Ti's, trying to squeeze a 7th card in doesn't even post. The screen looks and behaves like a NES cartridge with dust before the bios can even load. Let me know if you make any progress.
i had to change the bios settings for pci-e speed to Gen2 for the three pegs, for it to post..
I also had to change to gen2 for cudaminer to be stable with 6 cards under win7. Interestingly enough with 6 cards on gen3, one card would always under perform, while the other 5 would have hashrates from 350-400. The CPU would choke on all the interrupts and everything would eventually crash -- but it was nice to see hashrates that high, even if they were exaggerated. Now that I am stable on gen2 though, maybe I'll try that 7th card.
The best cudaminer config for this type of setup is: -i 0 -l T5x24 -m 1 -H 2 -b 4096