Many motherboards now come with 24 pin power connectors, This came about as the PCI-Express bus requires more 12volt power than supplied by the 20 pin connector. In cases where a motherboard is using onboard video or the PCI-Express video card is an entry level card a power supply with a 20 pin connector may fine. A 20 pin power connector fits into a 24 pin slot by offsetting it so that 4 pins on one end are not used (only goes in one way). Refer to the motherboard manual as it may give additional recommendations. See this article for a detailed explanation of ATX12V 2.0.