You're right that you should use the 480x320 viewport size, as all sizing measurements are based on logical pixels not the screen's physical pixels.
If you want to thoroughly test the site for retina content, and you don't have a retina screen, change the value of layout.css.devPixelsPerPx
in the about:config page to 2
. -1
is default. Note the UI will all look huge, but this will properly kick in pixels per px media queries, etc.
I just tried the devPixelsPerPx tweak along with RDV, and it works well - RDV looks like a great tool!