Direction needed for first PDA

Hello all - Despite considering myself a gadget-man, I have never owned one of the PDA's. I am now very interested in some birding software, which runs via a PDA. I also have sat-nav, but would like to have a second satnav setup.

What is the difference between the mind-boggling array available? - what spec might I require, assuming I might want phone capability, bluetooth, mp3, mpeg4. I hate the idea of purchasing at entry level, then having to upgrade soon after.

Any advice accepted (be gentle with me though) or any pointers of where to look for the information.

Steve