Hardware :: Cost Efficiency Of A Computer PSU Based On Watts Rating

Can someone tell me if there a difference between running a 1000 watt computer PSU and a 1000 watt light bulb? For instance, Im sure if you burn a 1000 watt light bulb you'd burn the maximum amount of energy all the time (1000 watts). Using the full 1000 watts might be substantially more costly at the end of the month if you didn't need all the light produced all the time, but of course as long as the light was turned on it would burn the 1000 watts.

Now my question about a computer, while 1000 watt is probably an overkill, I'm thinking I should get a bigger wattage PSU than my current 400 watt power supply. If I got a higher wattage PSU, is there a chance that the wattage rating would burn the power based on the immediate computer load (components installed and intensity of the immediate applications) rather than a constant use based on the 1000 watt rating? During off peak times when I might not have many USB devices plugged in and not be running graphic intensive applications, would it be possible that the 1000 watt PSU would be less bucks (or equal to) the cost of the 400 watt PSU? Or would the 1000 watt PSU cost the same amount for the 24 hour operation of the computer as running a 1000 watt light bulb for the month.By the way, I'm looking at purchasing an 800 watt PSU and I'll be upgrading my 88000gts graphics card soon. I just replaced the case with an AXP 600 that has lots of room and cooling facility. I probably don't need the 800 watts at present, but with time I will be replacing the CPU and motherboard.Is there a chance (or scenario) where a higher wattage PSU might actually be more cost efficient at the end of the month and possible burn less electrical dollars because of less strain to keep up?