Hi mate - I'm not convinced by the add-on/interface ones that link to palm top/lap tops... Specifically, because it's all a bit of a faff: you end up driving into a wall whilst fiddling with Ipaq keys and/or thumping the interface.. The 'TomTom' has one of the stupidest names, but is a 'proper' in car system...
Ideally, you need one that uses DVD's as the source: I trialled one the other week, and was mightily impressed: it gives voice cues, can navigate to an entered post code, and splits the screen as you approach, say, a roundabout, and tells you (graphically) which way round to go and which exit to take!!
I've used them from military systems back in gulf war one to this thing last week, and for 'normal' in car use, have concluded that you need to pay the extra for a good system: safety and utility are compromised in the less dedicated sets, which are technologically clever, but not suited to the purpose... What you want is something that simply provides clear audio/graphic cues for minimum effort (eg a street name or postcode followed by a button saying 'get me there'). The best 'factory fitted' units include inertial gyros, and mix input from the GPS and the gyros to further improve performance.. Sorry, bit rambling, but what I'm trying to say is that I'd not mess about with linking a GPS engine to a handheld via some more or less competent software, but would bite the bullet if I actually wanted a useful and safe system..
Incidentally, just for fun:
The signal strength at the receiver is less than the energy generated by stroking a cat's fur
Think of the computations required to fix your position in 3d on a spinning ball (that is not a perfect sphere) by measuring the time delay and doppler shift in a signal triangulated from at least 3 satellites following their own independent orbits...
The Americans gave up on the military/civil accuracy differences when a British surveyor realised that you could 'edit them out' with a stationary gps connected to a computer on an ordnance survey trig point...