EVERYTHING hooked up to the mobo, or usb will draw power (watts)

find the total wattage your system needs, and get a power supply that can handle that much (personally, if my system needed say, 120watts, i'd go for a 160watt power supply, even though they have 120watt supplies, just for future additions if i ever need them, then no need to buy another supply :T)

but if ur cpu/mobo only need 70watts, ur gonna be hard pressed to need anymore than a 160w at best, even with multiple USB things, the hard drive and screen