This article from Tom's seems to be the place to look:http://www.tomshardware.co.uk/gaming-graphics-card-geforce-gtx-480,review-31866-3.html
The Radeon HD 5770 seems about to be about your price range.
The official recommended power requirements for that card are as follows:
450 Watt or greater power supply with one 75W 6-pin PCI Express® power connectors recommended (600 Watt and two 6-pin connectors for ATI CrossFireX™ technology in dual mode)
This applies to the whole system not just the card.
So realistically, if you don't plan to Crossfire in the future you could get away with a good 500-550W PSU. Corsair, Seasonic and the newer Antec models are the recommended brands.
Off the top of my head, the Corsair 550VX would suit your needs fine, with some headroom to spare.
Hope this helps!