Are you really sure you going to use such a thing? I tried carrying a PDA around for a while, but just found it pointless after owning a Palm III, Palm V, then an iPaq. My iPaq is likely dead now, having not even been turned on in 18 months. PDAs definitely have their use, but also I think were a bit overhyped. Wireless internet anywhere on a small screen seems mostly pointless to me, since computers are usually easily in reach, or Google SMS can answer simple things like movie showtimes or local restaurants. I tested an OmniSky modem on my Palm V, and found it more novel then useful.

Contacts, I have them all in my RAZR phone, and maintain then via iSync these days, and have contacts still entered in my first Nokia thanks to SIM based contact storage. With a bluetooth based program, I can ensure my phone is synced once a day when I am in range automatically. No need for a cradle, no manual syncing, no forgotten contacts.

Calendar, I have iCal for the rare occasion I need something scheduled. The calendar then gets synced to my phone along with the contacts via the already mentioned iSync.

Files, I have a USB thumb drive that sometimes comes with me, but usually I just find it easier to have web based IMAP mail and web space.

GPS, I still have my separate Garmin GPS V unit that gets put in the car when needed on vacation, or when geocaching. For local use, I find it's not really needed, but that probably comes with knowing the town inside and out.