or ~20 in each 12v rail if you have two.
Except it doesn't work that way. My PSU has two rails, each individually rated at 22A. According to what you wrote above, I should be fine. But the PSU can only provide 34A altogether, which falls short of your 40A mark.
The best thing to do is find out how much power you need, and then buy a PSU that can reliably provide that power and more. If you need 200W under load, don't buy a 650W PSU. If you'll be CF/SLIing, make sure you have enough for your cards and the right connectors.