I gave a quick overview on what you can find on the market regarding PDA devices, sadly the PDA itself is almost dead (due to the smartphones).

I think the iPod Touch is your best bet atm. The iPod touch (specially the new announced model for $199) probably is going to have better support and require less hacking than the N810 to do what you want. Official support for the N810 has ceased and independent developers are more interested in the N900 now.

From what I know, you can use Google Docs, Calendar, and other apps in the Touch. No video calling, though because there is no camera, and you may need to Jailbreak it (Apple walled garden, iTunes, etc). No multitask as well.

Good luck!

P.S.: Have you checked up the N900?