First figure out which card is newer, then look up reviews that hopefully include the older card.  GTX480 appeared in the Spring 2010, while HD7790 appeared three years later in Spring 2013.  I looked up a review for the HD7790 by guru3d.com mainly because they tend to include a plethora of older cards for comparison, since not everyone upgrades to every series.  So give their results a look through for a title that matches what you generally do and make an estimate.  For the most part, you're right: The GTX480 is faster, but the gap isn't wide, I'd say an average of 10-15% slower.  Except the HD7790 uses half the wattage.