A single GTX580 is a much superior card, that will need a 600w psu with 42a on the 12v rails plus one 6-pin and one 8-pin PCI-E power lead.
With two gts250 cards you probably have that.
Here is a good one that is priced well below your budget $475:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130615
GTX570 in sli will have better benchmarks, and fit your budget, but I really doubt you need that unless you are running a 2560 x 1600 monitor.
For sli GTX570, you are looking at a 850w psu.