Craig, I am about to make the full circle, 650 to 400 to 650. As far as the highway stability and added weight, I have I can not remember a time on the 400 when I felt unsafe due to the weight or stability at high highway speeds. You will gain some weight for high speed riding with the 650 but you will also gain the weight for pushing around and low speeds. Also, the drive train on the 650 makes it a lot more difficult to push around. So, I guess it depends on your around town to road trip ratio. Unless you are going to spend a lot more time on the highway than in town you may want to consider staying with the 400 for a while.
If you are riding a lot two up on the highway, like Jim, the 650 would work out better.
Another factor is your inseam. The 650 has a higher seat height. It bothers some people, others prefer to have their feet more planted when they stop.
Pushing the 650 around, the added weight at low speeds, and seat height are some of the reasons I wanted a 400 and traded my 650.
05s, no one knows yet. We should find out in a month or two. If you really want the same color blue as your '03 400, you can more than likely find the same color blue '03 650 around some where, if you are willing to pick it up or have it shipped.
Ideally I would keep the 400 for around town and weekend day rides (100 to 200 miles) and save my 650 only for long trips only. But, I cannot see having that much money in bikes. Maybe I can find a 150 to 250 cc scooter cheap for around town. I would like to have another Helix or a Reflex but they seem to too high for a second bike and my budget.
I am moving back to the 650 for the smooth drive train operation, long trips, and my personal hang up about been able to out perform or at least give a good showing with most bikes on the road.
While the 650 is a very nice all around bike. Like everything, it does a have some "less than ideal" aspects.
Finally, get the 650, if you are like me, you just want another bike to ride for a while.