Hi again,
Yes the OS is Flashed into the device, but my Qtek900 came with WM5 and now carries WM6 so I think that if You can run Win CE in it then You should be able to upgrade it.
Upgrading is an automatic warranty void action, so be carefull.
I can point two sites wre You can get more info on the subject if you wish..
If You got Your mind set on a new PDA, The main specs are Processor speed, Ram mem ( this is waht is most often used by the system and the users to store volatil info and other apps), Rom mem( this is were the OS is flash to, so any 64Mb will do, but HP has one with 1GB used to store the GPS sofware and maps, with built-in GPS receiver), and don't forger the comunication capabilities such as Bluethoot, Wi-Fi and Infra-red( Not really used nowadays), and the external storage as well, Sd, Micro-SD, Memorystick, depends on the manufacturer...
Hope I've helped you.