my crappy old monitor supports all the resolutions, but 720p is a little shaky (can't tell in games, but can tell in xbmc). I cant remember if my 1080i image looked good or not, but for the most part 480p, unless you're on a really big monitor is fine for XBMC, and then only a few games support 720p, and even less 1080i.
So, its not really an issue (other programs like emus are displaying lowerquality graphics to begin with so 480p and 720p are fine for them too)
plus, on a side note: who really is going to take the time while playing to notice the difference. I mean, the game is in motion, your not 2 inches away from the screen, and i doubt any of our eyes are 20/20 even with glasses on. 480p is awesome, and 720p is even better, but just enjoy the stuff, whatever works for your setup should be fine.