In addition to being limited to which NVidia video chipsets can be used together in SLI mode, according to which main chipset the mboard has, the capabilities of the main chipsets varies.
To use two video chipsets in SLI mode, both video chipsets must support SLI mode, and the main chipset must be either a Nvidia main chipset or be one of certain Intel main chipsets. You can't run any video chipsets in SLI mode on an AMD main chipset mboard, and you can't run any video chipsets in CrossfireX mode on a NVidia main chipset mboard.
You may or may not need to connect the SLI cable between SLI capable cards for their video chipsets to be running in SLI mode, depending on which main chipset the mboard has, and if the main chipset doesn't require the cable to be connected, SLI mode may NOT work properly when you connect that cable.
cbF has/had a mboard with an Intel X58 main chipset.
It can support two (or more ?) of certain specific video chipsets in CrossfireX mode (most if not all are ATI video chipsets) or two (or more ?) of certain specific video chipsets in SLI mode (most if not all are NVidia video chipsets).
Some Intel main chipsets that have onboard video, e.g. Z68, support
LucidLogix® Virtu (Universal Switchable Graphics) - Auto Switching between Integrated Graphics and NVIDIA/AMD Cards
E.g. A review about LucidLogix® Virtu
I don't know which NVidia video chipsets can be used together in SLI mode, but here's a chart that shows which ATI video chipsets can be used together in CrossfireX mode:
CrossfireX Compatibility chart (latest) on this page:
Click on it for a larger image.
Some of the Intel main chipsets support CrossfireX mode - some of the mboards that use those are shown here:
AMD CrossFireX™ Certified Motherboards