How to search for motherboards for 2+ graphics cards?

Hi,

I'm finding it hard to search google for motherboards that have more then 2 graphics cards, I can find things like:

Expansion Slots 
2 x PCI Express x16 slots, running at x16 (PCIEX16_1, PCIEX16_2) (Note 3)
2 x PCI Express x16 slots, running at x4 (PCIEX4_1, PCIEX4_2)
2 x PCI Express x1 slots

It's my general understanding to search out mobos with 16x slots is this right? if so is x16 running @ x4 acceptable? Any search tips or links would be much appreciated

Thanks,