Thanks for your assistance. I changed the power supply to a 450W and it still didn't work.
I believe the problem was actually that the card was a PCI-E 2.1 and my motherboard has a PCI-E 1.0 slot.
The PCI-E 2.0 cards work with a PCI-E 1.0 but the 2.1 doesn't. I changed the card to a Nvidia GT 430 which was a PCI-E 2.0 and it worked fine.