As tecmo34 already pointed out, it would be helpful if you could post the make and model of your power supply. I am wondering why your power supply does not have at least one 75 watt, 6 pin, PCI Express power connection.
EDIT - I just read your other thread. I think you heard wrong.
ATI Radeon™ HD 5750 System Requirements
PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
450 Watt or greater power supply with one 75 watt, 6-pin, PCI Express® power connector
600 Watt and two 75 watt, 6-pin PCI Express® power connectors for ATI CrossFireX™ technology in dual mode