I have a 550W PSU from Chieftec. It has three 12V rails, 18A each (3×18=54A). Now I want to buy a Geforce GTX 260 that requires a 500W PSU and 36A power on 12V. I suppose my PSU will be fine, these 12V rails should self adjust ?? Funny thing is, the PSU only has one PCI-E cable with two 6-pin connectors.
You may be fine, but you can't add the rail amps. Also, if one rail is "dedicated" to the PCI-E power, than it can easily be overwhelmed. I know my old 2900XT (which drew a bit more power) did that to my 550W. Quality and age of the PSU is also an important factor.
Your psu does not have combined 54 amp 54x12 = 648 which is more than it can handle. You should try to get the rating of the manufacturers website if combined 36 you are fine.
His power supply has exactly 54 Amps on the 3 +12v rails combined.....you add the max current of each rail 18+18+18...I don't know what kind of electronics theory you're using that you multiply his +12v current (54) by 12 (his voltage).....that would give you a theoretical max wattage output not current.