Wrong.You don't add the rails together, there is an equation for this....Linkage:http://www.nvnews.net/vbulletin/showthread.php?p=1982571Essentially you divide max power for the rails by 12 volts to get total amperage.

It's true that adding the rails together doesn't give you the constant amperage delivery capability of the rails, but it's a close enough approximation as to be functional.

Looking at the two PSU's I have right now, neither of them show a max power - just the amps for each rail.The Thermaltake Toughpower has 4 +12v rails: 18A, 18A, 30A, 30A. The box says combined loading of 62A. I assuming that this is based on the formula mentioned in the link. Since the video card I am looking at recommends 550W min and 40A on the 12V rail, it sounds like it will be more than sufficient. Glad I opted for overkill with my last upgrade.The second one I have is a Antec Smartpower SP-500. This one has two +12v rails: 17A and 19A. I'd like to put this in my secondary rig but my current video card says it needs a 450W PSU with 30A on the 12v rail. Without that max power figure, it's difficult to tell whether it will run the card well. Just doubling the amps gives me 36A but as the other post said, it is not an exact figure. I'm thinking it will be close enough.Thanks again for all the help. I think I'll be in good shape with what I have.

That Antec PSU should be capable of right around 30A constant delivery over the +12V rail. 36A is the peak delivery, constant should be 28-32A depending on temperature and aging factors of your particular unit.