Looking outside the obvious answer of the initial question in that any piece of string is twice as long as half it's length; why not buy a power consumption meter? You plug it into the socket and then plug your PC's mains cable into it and it will give you an LCD readout of the consumption in watts.
Of course, have a power supply rated at 500W means simply that; it is rated to supply a maximum of 500W under full load. That in no way at all means that it will consume that amount of electricity during day to day operation. As snags link points out in a table, the energy consumption of a PC will depend on both it's components and also it's usage patterns. Surfing the net and typing a word document will, for example, use significantly less energy than playing a modern PC game or encoding video. So really, there is no way to say exactly aside from testing it yourself and recording power draw when doing the things you do with it. Even then, the best you'll be able to work out is a semi educated guess.