I'd like to know whether there's a sufficiently advanced pocket PC (like the HP iPAC 5550) which supports the adding of a GPS antenna and GPS software. If only MIO 168 and few similar instruments support GPS, what's the best such pocket PC (are there any which support wifi, bluetooth and Windows mobile 2003)?
I have actually been looking into the same thing recently. I found a great website with reviews of all the GPS which you can add to Pocket PC, you can get Bluetooth, Compact Flash or expansion sleaves by the look of it.
My personal preference is towards a bluetooth device such a CoPilot Live 5, Tomtom, or Destinator 3 with a bluetooth GPS (can be any brand but most can be included with the software). The GPS will work with (reportedly) any Bluetooth enabled Pocket PC, such as the iPAQ 4150 or most of the new ones.
Anyway take a read of the website, particularly the software section which will tell you what you need to know. In answer to your question I would be inclined to look what other features you would like in your PDA before making a decision. A bluetooth GPS and software seems to be able to work on any bluetooth enable PDA with a SD card slot (which all have now days). Persoanlly I owned a iPAQ h4150... but recently had it stolen and am now looking at the iPAQ hx4700.