Definitely make the seat reservations now, especially on the long haul segments. Through Rail Europe, and from the U.S., seat reservations run $11 each - in Europe, they are 3 euros each. (that is per seat and per train segment; if you change trains, another reservation is required for that set of seats.) I seem to remember that you can't make seat reservations for travel within Switzerland, i.e. Geneva-Montreux, but not positive.
I'm doing a mid-June trip in France with the whole family, and some segments I wanted on TGV were sold out already, so I got on it in a hurry.
And, as everyone else pointed out, travel in 1st! Far more comfortable and quiet.