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 have 8g of ram (4 sticks 2g each), core 2 quad 2.5 ghz processor, 1 sata hd, 1 80mm led fan, and soon im adding this http://www.newegg.com/Product/Product.aspx?Item=N82E168...
At the moment i have a 300 watt with a onboard video card, and if i want to install the new video card im getting it said i need a minimum of 450 watts so i got 500 watts