Sorry if I wasn't clear enough. I know what the molex -> PCIe power adaptors are. What I wanted to know is, when are the PCIe extenders with attached molex (the ones cablesaurus sells, for example) needed, and when can the ones without molex be used.
The story I read was that some cards consume a lot of power such as the 6990, so much so that the PCIx slot cannot supply enough and so the molex is added to extend the amount of power supplied to the pcix port. Basically it's for the dual gpu single slot cards.