I have a Compaq iPaq, about 6 years old and run Windows Mobile 2003. I have it set up so that it syncs with my email, address book, and Google Calendar--so when I enter an appointment online, on my home computer or on the PDA they all sync up without (many) problems. It has an SD card slot, so expanding the memory is real simple, and it also has Word, Excel, Internet Explorer, Windows Media Player (plays music and movies) and Adobe Pocket Reader. Pretty handy little device, though mine came out before wireless Internet was big so I have to use a little backpack (which fortunately has an extra battery in it) and a PC card to get WiFi or bluetooth capability. Or, it can use my home computer's internet when it's hooked up to the charge/sync cradle.
I have also previously used a Palm Zire 21 and Handpsring Visor Deluxe. Although I haven't taken a look at Palm lately, when I got the iPaq Windows Mobile really had the advantage in terms of available software, networking capability and expansion. Now, you also have the option of the iPhone/iPod Touch or Linux-based PDAs. Unfortunately, that probably won't help you narrow it down any.
