please see this..forum http://hardforum.com/showthread.php?p=1037211058...its not mine but it tells to find out amperage on 12v rail we need to divide wattage available at 12v to the 12v to determine current on it...mine is and 450w on 12v rail that means
450/12 = 37.5Amps maximum so i am able to run hd 5670
You have 19A on your 12V rail, thats how much current it will give, it wont give 450W, not even close to it, 19A @ 12V is only 228W. The watts/12V to get current is only for when the unit lists the combined power of its 12V rails as a power instead of a max current, but for a single rail PSU your max current is the current for that rail, there is no math required.
The 450W rating is for all of the rails combined, it would be the power that the 12V can provided added to the power from the 3.3V rail and the 5V rail, and usually rounded up to a nice neat number.