I recommend the BT 77 gps receiver if you go the PDA + gps route. I have one and performance is excellent and no problems using with Tomtom.
http://cgi.ebay.co.uk/v2-Bluetooth-g...QQcmdZViewItem
Rough calculation: Bluetooth equipped PDA for about 120 + 45 (Gps receiver) add another 15 for PDA car kit = 180. Now all you need is Tomtom on a memory card and you have sat nav. Total price will be nearly the same as Tomtom one but you can do so much more with a PDA.