The ones on the top are for CPU power, at least one is mandatory for a system to run, but you have two cables so you can use both, but either way it is irrelevant how many videocards you have running in your system. The bottom one for supplementary PCI-e power, and it is optional, but if you have multiple cards installed it will offload the 12V line off the primary ATX 24 pin connector. For videocards you'll want to use PCI-e power cables, with VGA labels on the connectors. PCI-e power and EPS power cables use different connectors, you can't interchange them.