Since there will always be a better version next month, you need to want a month, then another and then another... I've yet to see an upgradable PDA. I have added a memory card or a wifi card, but such is not a full upgrade.

The better track (research?) would be to find the software you want to run and note what PDA runs it.

In my world, I'm considering the Axim X30 w/wifi. That way the data and apps would be on the network so as I upgrade the network I get a better PDA.