I like the 500's and 550's, neither seem underpowered to me until I ride double. I go 165lbs so does my wife, +. The 650 is reserved for wife rides around here, it just handles better under load and has enough power that it's a comfortable ride.
What I've seen for upgrades over the years was nothing that someone that doesn't work on these bikes would even notice. like everyone else has said , there hasn't been many earthshattering changes just minor improvements.
Like any other vehicle that you would purchase ,condition first,price second, especially if you aren't inclined to do the work , and you have a paying job,which would leave you in the " condition only" group The example Stev-o gave you looks like a real nice place to start to us, but we are all veterans of project bikes, our eyes are trained at this point to search for obvious things that we have experienced in our projects and are more generous with our idea of condition. Meaning , if you are looking for a showroom condition 550, be ready to spend $4500-7000, a really good rebiult $3500-5000, a low miler in really great shape $2500-4000, an every day beater in decent shape 1200-2500, something soon to be a project around a $1000, under a grand will need work. Anything less would be considered a project, unless it was in showroom condition-in which case it would be a gift from god or somebody"s pissed off old lady..
Those are the pricing levels that I've been seeing lately, hope it helps, good luck and safe ride.