dual 20 should be good make sure you have each rail in one of the plugs
if the psu is cross connected your find
the psu is too small for most systems you system is 400w and you have a 80-85% max
you want 60%
400/.6-.7 = 700
Are you psychic? The only thing you know about his system is the GPU (which is not terribly power hungry). A 500W PSU is plenty for most systems.
And you will not have the option of moving a PCIe connector to a different rail.