How many watts does a fan use?

It is more important to look at the number of A on each of the rails. Most PSU give the maximum W if all rails were fully loaded, so you could for example use just 400W and overload a rail, hopefully resulting in it turning off for a good quality PSU.

Also cheaper PSU brands can not sustain the value they give (or even realistically reach it ever in some cases), so average usage must be much lower (at a figure they often don't provide, instead just giving you the "peak" value at 20 degrees C or something).
 
I would not recommend a 500w power supply if your computer is using 450w.

It's better to use a power supply calculator than to try to measure the wattage of each component.

http://extreme.outervision.com/psucalculatorlite.jsp

Its not using 450 right now, its using 300 right now but the video card im gonna get is said it needs a 450 watt PSU so i got a 500 watt PSU.