no
single rail have unlimited power, quad rail is limited and most are cross conntect
v1 + V2 +v3 = v1
v1 + v2 +v4 = v2 and so on
if you have a quad rail, the max is 12v and most have a max of 18amp or 150watt some go up to 24w but that rare and may only be peak power
if you 9800GTX usess up to 250w
then the first 75w comes from the pci-e slot
then the next 150w comes from the first 6 pin - first on how its wired not in order you look at
the remaining is from the second 6 pin
8pin gives more and cross conected for hiigher amp on single rail
4870X2 can suck up to 350w+