I have no idea what the SLI-F that you're talking about is. There is only one MSI K8N Motherboard with 2 x PCI-E 16x slots, and that's the Diamond, which AFAIK, isn't available as retail anymore.
SLI stands for Serial Link Interface. It is Nvidia's technology to let you use 2 identical videocards in your computer at the same time. Performance increases range from 30% to 80%, depending on the game and how well it utilizes two videocards.
Since one [modern] videocard requires a PCI-E 16x slot, to SLI, you would need two of these slots. There are many motherboards out there with 2 PCI-E 16x slots. They normally cost more than those with only one, for obvious reasons. You do not have to use the second slot on these motherboards if you only have one PCI-E 16x videocard.
PCI-E 1x slots can not be used for videocards.