Consider how you'll use a PDA before you decide
An excellent question and one that begs a question for you in response.
How do you think you'll use a PDA to support your GTD workflow?
There is a tremendous amount of discussion on this board (and elsewhere) about the different approaches people take to using a PDA. Some do a tremendous of data entry directly into the PDA. Others (like me) tend to use it almost exclusively as a portable reference tool and prefer to do most data entry on the PC (or Tablet PC in my case).
I find I use the PDA to scan my lists, look up contact information, and very occasionally jot down a new appointment or task. I could probably get by with an entry-level PDA with no problem and a less-expensive option like a Zire would probably meet all of my needs. I have a Sony Clie UX50 which I acquired when I was making much greater use of the PDA than I do now and it's really overkill for me to carry such an expensive device given my current usage.
If you think you'll be using the PDA for a lot of data entry, reading of e-books and RSS feeds, listening to MP3s and audio books, and other intensive applications, a more full-featured unit with a larger screen might better meet your needs.
The software you think you'll adopt should factor in your decision as well. Do you need the larger display for working with Office documents, outlines, and other files?
I suggest you give some very serious thought to what your usage patterns will be before making a decision. I can say, from my own experience using both Palm OS and PocketPC devices, that the Palm is a better platform for me. My experience with the PocketPC was fraught with issues (I especially disliked the behavior of ActiveSync) and I was very happy to return to the Palm.
Nokia Series 60 Smartphone
Here - in Europe - GSM and Nokia are very strong. I am currently investigating the possibility to use the Series 60 smartphone (Nokia 6670) with external Bluetooth keyboard. The screen is small but the whole device is tiny and easy to carry around. It can be synchronized with Outlook.
There are also Nokia Communicators with QWERTY keyboard and bigger screen.
I recommend getting a phone/pda combo as well
This day and age it is often cheaper to get a phone/pda combo than getting them seperately. It is also considerably less pocket clutter to carry around.
Most of these devices include some dictation capabilities which can be augmented by the add in memory cards.
The Treo's are popular. I was stumbling through Amazon and came across a Motorola MPX220 unit that was $50 after all the rebates.
I recommend looking into the phone/pda combo. My preference is a devices that works with a stylus as the mini-keyboard devices cause me to fat finger too many things.