It doesn't matter if it's a PDA or full size. I myself use an iGo bluetooth keyboard (forget which model, I'll have to look it up when I have the time). It's a fold up one. Only problem with getting one is they're expensive, and the company has stopped making them so they're getting harder to find. I luckily got mine from a friend who wasn't using his tablet as much anymore so he gave me the iGo bluetooth keyboard.

You can also use USB keyboards (provided they don't require to much power) if you get that USB OTG adapter here or anything that turns on the n800's host mode.

Edit: I have the 5 row bluetooth keyboard.