No math to back up your statements huh? I didn't say you are a noob, just someone who is confused about things. I don't know why you are acting this way, but unless you can accept the fact that you might possibly be wrong, you aren't going to learn how/why you are wrong. If you're willing, read the rest of this post. If you are so sure you're right, either back up your claim with evidence, or show me where I made a mistake with my math. (seeing as you haven't done that yet, I'm assuming my math is right.)
Lets review. Each rail can output a certain amount of power. In the case of the SP 450W, the first 12V rail can output a MAXIMUM of 15A, while the second can do a MAXIMUM of 17A. Some people mistakenly believe that you can add these two number together to get the total amount of power for the 12V rails. (or in your case, 18A + 18A) The problem with this is that those are the individual rails, not the total amount of power for the PSU overall. The SP 450W can't output 15A + 17A on the 12V rail at the same time, but in fact 24A. Lets look at a good example. Click on this link, then the third picture.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371009
This PSU has four 12V rails that individually can output a max of 18A. If you follow the mistaken logic, you would think this PSU could output 864W combined for the 12V rail. (18A * 4 rails = 72A. 72A * 12V = 864W. See how I proved this with math?) Antec doesn't claim that this psu can output a COMBINED wattage that is that high. They say that the combined output for ALL 12V lines is 768W. 768W / 12V = 64A. This means that the circuitry inside the PSU can't supply enough power to feed the maximum that each rail is capable of. If divided equally, each 12V rail could have 16A. (in actuality each rail would probably be different. The motherboard rail might be 10A, the CPU rail might be 15A, the GPU rail might be 18A, while the fourth rail might be at 8A.)
Some PSU manufacturers don't make it easy to figure out how much power the PSU can output. They might list peak power instead of sustained. They might just list each rail and the amperage, with no total figures. These are the power supplies you should avoid.
I'm not claiming that you misread your label. I don't doubt that it said 18A under both rails. But to write what you did, "18A + 18a = 264w maximum combined wattage on the 12v rails" is wrong. 18A + 18A = 36A. 264W / 12V does equal 22A. 18A maximum for each line, but combined, it could only do 22A. (either 18A on one with 4A on the other, or 11A on both, or any number of possible configurations.)
I hope this makes sense.