Is this a good PSU for the GTX 580?

Looks good. you need one 8 pin and one 6 pin for that GPU, and the CX has two 6+2 pin connectors.

Note the cx line is an entry level psu. Not designed for high end systems that will be overclocked, just something to keep in mind. But it has enough watts and amps, and it has enough connectors for your gtx 580. Good luck!

This one is higher quality and cheaper
XFX 650W PRO650W Core Edition Single Rail $59.99 after mail in rebate
http://pcpartpicker.com/part/xfx-power-supply-p1650snlb9