your better off using the new power supply and using the old one as back up. with any power supply the devices you uses (ie mb/video card will have the same load on a 400w unit or a 800w unit. it how much amps the devices together draw that going to set the wattages uses of a power supply. with any power supply your better off having it slightly oversized then under. with your old power supply look at the 12v rails and see what it rated. take the cpu wattage and the video card and add them together. that going to be the main load in the computer. with old power supply have to be careful that the caps not dried out or leaking. (newer power supply some have thermal sensing so that the larger 120/140mm fan will spin up as needed.