Specs on the Corsair CX430 ($50) are,
Voltage +3.3V(20A, 120W) +5V(20A, blank) +12V(28A, 336W) -12V(0.8A, 9.6W) +5Vsb (3A,15W)
Total Output 430W
Basically the guy said you look at your amps, not the total Wattage, so your +12V is your video card, +5V is your peripherals. OK Very helpful, I guess.
Time for a little bit of Ohm's law theory...
Here is the general formula for Ohm's law: V = I x R (or V = A x R). Power is a combination of Voltage and Current, so to calculate power P (or W) = V x I (or A). Simple algebra allows one to play with the formula to solve for either value.
Based on the specs of the power supply you quoted...
+3.3V(20A, 120W) ( 3.3 x 20 = 66W actually)
+5V(20A, blank) (5 x 20 = 100W)
+12V(28A, 336W) (12 x 28 = 336W, got this one right)
-12V(0.8A, 9.6W) (12 x 0.8 = 9.6W, also right)
+5Vsb (3A,15W) (5 x 3 = 15W, right again)
So now add up all the power (Watts) = 526.6W?!? How'd they say it was a 430W supply?
Here's how they come to that number. The maximum total power available for the whole power supply is approximately 430W (most likely a bit more, I'll explain that later). When you put a load on each of those rails (or voltages), each of them collectively will draw power from the primary side of the power supply (the side where the 120VAC goes into).
Let's say the 12V rail is drawing full current, at 28A, and the 5V rail is drawing 3/4 of it's rated current at 15A. So then 12 x 28 = 336 W and 5 x 15 = 75W, so a total of 411W is being pulled out of the supply. Given that it's rated at 430W, that'll leave 19W available to be drawn by the other rails. If you tried to exceed that remaining power, the whole supply will start to "fold back" on all the rails, meaning that all the rails will drop in voltage to compensate for the extra draw.
The 430W rating is determined by the actual power rating - 10% for performance/safety reasons. It's likely the average Corsair 430W power supply is capable of supplying up to about 480W. With varying values of resistors, capacitors, inductors and transformers, as well as the diodes and voltage regulators, a 10% allowance is typical. Also, if you were to load down the power supply to 430W across the rails, you would see that you are actually drawing more than 430W from the 120VAC line. This is loss due to inefficiency of the power supply and also a loss in the heat generated. Switching power supplies (the ones usually used in PCs) are typically more efficient than standard power supplies (transformer, rectifier, filter, regulator).
Hope this helps in understanding how power supplies get their ratings. As for the 12V1 and 12V2, ExDilbert is right. The secondary side of the power transformer would have 2 coils for each of the 2 12V rails, and each would operate independently of each other, up to the maximum rating of the whole power supply.