Well after I posted this I realized I should just download and read the 605 manual. I found out it can also take .gpx files that the 305 can't and sounds like it will do what I'm thinking of.

It does suck that this costs more than my Garmin Nuvi 360, yet doesn't include the basemaps (or the bluetooth, 6-million point of interests, voices, etc.). I realize that Garmin and all GPS companies pay a fee for each unit sold to Navteq or Teleatlas for the maps, but it does seem that the map should be included at the price. Then if I want to tour Europe I'll go buy the SD card for there.

On the 305 I make a course (.crs) in Mapitpronto.com (pick your favorite site) and upload it. Sometimes I make courses randomly, other times I use the riding club cuesheet to plot the upcoming weekend ride course.
I'd expect the same on the 605/705, but it would be nice with turn by turn, stating the street name directions. Even without however, being able to see the course on a detailed map would still be much better than the generic black line on the 305.