m'eh, I personally wouldn't spend my money on something with a 5 inch screen. You can get older tablet pc's with 10 in screens for like $200. Windows XP, USB, etc....seems a little more practical for carputer use. Hell, you can even pick one up with a Reflective outdoor viewable screen - complete tablet pc, cheaper than the price of a new Xenarc.
Sounds like your PDA works for you, and that's great. I just require more than 4 gb space. This thread reminds me of the thread someone started, talking about how great their cell phone was, and how it can do everything a carputer can do.
Overall it's a great idea, just I like the larger screen. Built in bluetooth and wifi is nice tho on your PDA. What OS are you running?