I recently switched from a conventional PDA to a smartphone... a Treo 600. Gives me PDA and cell phone capability, and I wouldn't be without it any more. Instant access to my entire address book, which is currently 2800. Once I get the office list again, it will shoot up to around 7000. Only downside for that many is the time to sync up
Handspring/Palm just came out with a Treo 650... so look around for deals.
If I were doing it again, I'd probably get one of the Kyoceras (I think)... they're also smartphones, but they have the ability to use Graffiti for data entry. I can only use the thumb keyboard, which is nice, but not overly fast.
Having said all that, what do you want to use a PDA for? That will dictate the direction you go to a great extent. I use mine exclusively as a PDA. I'm not interested in using it as a mini-PC, with word, excel, and so on, so a Palm-powered device works great for me.