Ehm... resistors ain't a good option to lower V. Sure it does the trick but for it to work it creates heat. For shorter intervalls it'll work but when you're driving a motor for longer intervalls as with the fan you will most likely burn the resistor (something you don't want to happen while the machine is rendering stuff while you're asleep).
Best option is to find 5V on the card or take it from the motherboard.
