That's not how it works. The size of the card does not reflect relative power draw. Also, successive generations of cards do not steadily consume more power for they must stay within a standardized ATX power envelope. Even the dreaded GTX 400 series of cards complied with this standard, they were only seemed worse because of the bad transistor design within the chip's architecture which caused them to draw a bit more power under load than expected. You could easily run a new GTX 580 on a quality 500 watt power supply and not have to worry about overdrawing it or anything.
That said, a wonderful power supply for this build would be the Antec TruePower New TP-550, which is currently marked down at $69.99 on newegg. At this price it's a steal compared to anything else in this wattage range and is a very strong performer with some modularity as well.
Link:
http://www.newegg.com/Product/Produc...82E16817371020