Consider the Handspring. It allows you to plug in modules to add extra functionality to the usual diary/address book funtionality. There is a plug in GPS module and MP3 player and loads more. I would recommend that you make sure that you can install a MIDP Java runtime on what you buy, it looks like this is the way mobile devices such as PDA's and cell phones are going (has gone) as far as nifty apps are concerned. The palm will enable you to do this.
???pax