The 430W is the maximum rated OUTPUT power that can be supplied to the system. In no way does this mean the power supply is providing 430W of power all the time. It is providing only what the system is drawing.
This does get more complicated though, because the power supply is not 100% efficient. The highest efficiency I've even seen quoted for a computer power supply is 70%. Most computer power supplies are probably going to fall between 50% and 65% efficient. For a 65% efficient power supply that means if the system is consuming (i.e. power supply outputting) 100W, the power supply itself is consuming 154W. (100/.65) If it were only 50% efficient, that would be 200W.
I'd say the typical gamers PC built within the last 6 months is going to consume about 200W - 300W from the power supply, meaning it will draw about 500W to 600W from the outlet. Remember to add about another 80W - 150W for the monitor, depending on it's size. Speakers and printers (especially laser printers) can also consume significant amounts of power.
Power=Volts * Amps. It also equals Volts^2 / Resistance (ohms) or Amps^2 * Resistance (ohms). The easiest way to determine power consumption for a computer would be to use an multimeter and measure the amount of amps drawn from the outlet. Multiply amps by 120V to get Watts. (Please don't do this if you don't know exactly what you are doing. Amp measurements must be made in series with the load. If you just insert the probes into the outlet the least you'll do is blow the fuse in the meter!)
_____________________________ _______________________
You've actually hit on a very timely issue. Power consumption on the newest computer components is a major factor in why we haven't seen the same kind of advancements this year as we did over the past few years. (Excessive power consumption is also the reason why the next P4 processor release is not scheduled until November/December.) The amount of power consumed by a PC can vary wildly depending on their configuration. Newer computers can easily consume at least 2x the amount of power that the typical PC did only 4 years ago.
The current 3.2 GHz 'C' type P4 alone dissipates about 90W of power BY ITSELF, and the next P4/P5 (3.4 GHz Prescott) to be released is quoted at 103W max. The top end Athlon XP 3200+ is close behind at around 80W. The new Athlon 64 model 3200+ is lower at around 65W by virtue of being built on SOI wafers. (For comparison, a Pentium 2 400 dissipates only about 25W-30W.) The same is true of top end video cards. A GeForce FX 5800 or 5900 series card will dissipate almost 80W. A top of the line Radeon 9800 series card isn't quite that high, unfortunately I don't know of the top of my head exactly how much power it dissipates, but I'd guess around 65W. That's at least 2x the power consumption of top end cards from a few years back. I'd expect to see processors consuming up to 150W by themselves by the end of 2004.
Intel recently has been showing off the new BTX form factor for cases and motherboards. The new BTX standard moves components around to improve airflow and cooling. For example, it moves the CPU to the lower front of the case.
