Just to be explicit about it, 20/20 vision corresponds to visual acuity that can distinguish features subtending 1 minute of arc. The angle subtended by a pixel at 326 ppi is arcsine( (1 inch /326) / 12 inches) = 0.87 minutes. http://en.wikipedia.org/wiki/Snellen_chart#.2220.2F20.22_.28...

But I think the psychophysics get more complicated when you throw in time and movement.