Bizman, if your +12v rail is putting out close to the amps needed by the graphics card, it will boot and depending on the card you may just get a low power warning and/or the card will cause some weirdness with the system. on the other hand if the +12v rail is putting out ~1/2 the power needed, the card won't power up.
in Kenne's case, his card needs a minimum of 30 amps continuous and his dual 12v rail power supply is only putting out 17 amps on one rail and 18 amps on the other. if he could get ~25-26 amps on one of those rails, it would at least power up.