Open up a side panel of your PC, look inside. There should be a large cubic object that the main power cord plugs into. Somewhere on that cubic object should be a sticker that lists the wattage, record the number and let us know here!
its 600W. But I've got another question. I just bought the game ArmA2, and my graphics card should handle it very well, but when I'm (for example) looking at a village with some AI's in it, the fps starts dropping increasingly. Is this due to the processor? Because it cant be the video card..