It is the combined amperage... think of it this way, amperage multiplied by voltage gives you the wattage. If it was requesting 26A on a single rail for your graphics, that would mean youd be drawing 312Watts just on your graphics card!!! which obviously isnt right!
even then the rated minimum power supply is usually with a top of the line quad core machine and a few HDDs, so aslong as your close to that you shouldnt have any issues.
Multiple 12V rail PSUs are tricky, let me see if I can help you out.
First, to get the total 12V power, you CAN'T just add them together. Whats listed on the side of PSU is what the 12V rail can handle by itself, you need to take into consideration the part in the PSU that is generating the power. If you have a PSU that shows two 12V rails, and each is listed at 18A, that does not mean you have a total of 36A. Combined, you might have as much as 30A, or as few as 22A.
Second, most "multi" rail PSUs are actually single rail PSUs, with "tricks" inside the PSU to make it look like its a multi rail unit. I can assure you that the power is generated at the same time for all the rails.