Considering that the 21.5" iMac is current 102 dpi, then straight doubling would be 205. And I really doubt that many would be able to tell the difference between 205 and 218...
Not me, just purchased, the entry level I mac at the Apple refurb store, june 2014 model for 929.00 shipped. Should be here Friday, my 1st I mac, have and have had a few Airs, which I like.
Retina is nice but for now this rig will do me just fine.