Preference>Units>Pixels, or control click on the ruler.
Or File>Document Setup>Units>Pixels.
Or just use points and set the Artboard width to 580 points: As far as Illy is concerned, pixels = points.
I am using cs6 and the page with many graphics saved as ai measures 580 points. Will that work?
It will work so long as you are saving/exporting as a raster image, at a resolution of 72 PPI.
Points is an actual measure. Pixels is not. Illustrator's rulers represent actual measure, even when they are set to "Pixels." The actual measure that Illustrator assigns to a "Pixel" is 1/72 inch, which is the same measure as a point.
So if Illustrator's ruler says your page measures either 580 Points or 580 "Pixels" wide and you export that page as a raster image at 72 PPI, the resulting raster image will contain 580 pixels per row. But if you export that same page at, for example 144 PPI, the resulting raster image will contain 1160 pixels per row, even though Illustrator's ruler says it "measures" 580 "Pixels".