Wait, so I can only put the 20 pin into the 24 pin slot if I'm using on board video? Couldn't I just put it in and plug the 12v rail and the pci-e card in with the other cables?
Yes, if the PSU has PCI-e power plugs. But most 20 pin PSU's are old enough that they do not have the 6 pin power plugs. And you cannot use a 4 or 8 pin CPU plug. They are wired wrong.