In reply to Gone for good:
Fwiw, which probably isn't much, I started looking into this a while ago (and there's a brief thread on here about it where I asked a similar question). Of the Garmin's I came up with this list, the only thing I was really interested in was navigation:
800 ********* good nav
820 **** crap nav
Edge explore 820: same nav issues as 820
605 ******
520 ******** maps not great
1000 edge and edge explore: great nav
20/25: basic nav
Edge touring: decent nav and road vs off road
So basically Edge touring gives you decent enough nav for a cheap price, otherwise you're looking at 800 or 1000 series. No idea about HRM or other fancy data measurements (that I am very sceptical are ever of any use to anyone other than Tour de France riders) but you might be able to build on the above.