I think your best choices would be a 5770, a 4890, or a GTX260 depending on whatever best price you could find. None of these would require an upgraded power supply.
Guru3D says the card will require at least 38 Amps available on the 12 volts rails.
If this is the PSU the client has, he has a total of 19 amps on the 12 volt rail:
http://www.newegg.com/product/product.aspx?item=n82e16817171007
He cannot power a GTX 260 or any comparable GPU. We don't even know if his PSU has PCI-e cables! You sure as shit don't want to run this thing off molex adapters.