The HD 5670 is a good option. The HD 6670 is the latest version so it is a little more expensive, but both are about the same GPU.
Both these cards are the fastest available without using a PCI express power connector , which means they get all of their power from the motherboard pci express slot. This is about 75 Watts max power that they pull.
As jaguar said, a min 300W PSU is recommended since the rest of the system could easily pull just as much or more. Some manufacturers power supplies are woefully inadequate for upgrades, in particular Dell and Gateway are known for putting 200W PSU or less in their PCs.