As a Palm owner, I have learned You usually can't update the OS. It's stuck at what you get when it comes from the factory. Pocket PC's and some stuff that runs HandHeld PC are the ones that can be upgraded. The last Palm I used was the Tungsten E. It had OS 5 I think. I got an HP Jornada 720 because the Palms internal battery didn't last over a year. Not exactly vintage, but a lot more usable. The Palm company. (In this case Palm One) gives you some money off a new Palm if you send them your old one. It's not much usually, but its a nice discount off a new one.
-Vlad