but only 16 PCI-e lanes...
The GPU already uses 16 lanes and then the samsung needs 4 lanes makes 20..
no, 16 is the max that goes to PCIe expansion slots (that MB has 3)... the rest of lanes are not going there and can be used for NVMe, etc... so if you put 16x PCIe card in one slot then you can't have any other cards in the other slots for that 16x card to work in 16x mode
