I bought a HP Jornada 928 PDA with built-in TomTom navigation plus a post-code and radar finder for about £350 on t'eBay a month ago. The sat nav system is excellent (you can see pics on Guydewdney's
totalPDA link) - better than the £2k DVD sat nav system I had in my 2002 Shogun 3.2DiD - but you also get the usual PDA benefits
and you can insert any sim card and use it as a mobile phone!
It came with a decent GPS receiver and all I've had to buy is a mount for a tenner (some eBay listings already include these). OK, there's a few wires hanging about but I can move it from vehicle to vehicle in seconds.