Atx power supply model ax550w/p4a???HELP?

Can i use the ax550w/p4a power supply in a old computer? it has only 20 pins and the other power supply is a dell nps-250db b 250w i think that on has 24 pins the mother board has 24 pins is ther a way i can use the ax550w on that mother board need more power for a new video card.