Here's a couple things to think about, though I'm by no means a PDA "enthusiast." I don't really know the current state of the market very well, but these are some general principles that may help.
Like computers, different models have varying processors and memory. The need for these depends a lot on what you'll be doing with it - more speed means you can run more complex and powerful programs. If you're just going to keep a calendar and an address book, then this doesn't matter at all. As with laptops, consider power use - color screens take up more power. Also, some cheaper models run on AAA batteries, which don't last very long. Then there's also the whole internet connectivity issue, which I don't know a whole lot about.
Personally, I use the Palm m500 - more power than the entry-level Palms, b/w screen, no internet stuff. I'm a teacher, and run a couple programs that set up customizable databases for grades, contact numbers and logs, etc. along with the usual calendar and address/phone lists. Even on the slightly more powerful processor, the programs take a little time to run and search - for me, the cheaper, lower-speed models would take too long to be working with while talking with parents or working in the classroom. I don't need the color screen for anything, and the b/w screen along with the Li-Ion battery mean that, if I'm really lazy, I can go close to a month without recharging. Mind you, I don't play Solitaire on it in my free time either - just business, thank you. I absolutely don't need to pay a monthly fee to get crappy, low-res internet or mail access, so the expensive Palm VII was out from the start. All in all, the unit works exceptionally well for me - well worth the ~$300 or so I paid for it.
Hope this gives you some ideas.
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