I would recommend a PDA version or if you have a smart mobile phone like 6630, 6680, N70... then try TomTom Mobile.
I preffer the PDA version just because you can do more with the PDA the TomTom all in ones are too expensive for my liking, for the same price you can get a half decent PDA or a smart phone or a PDA Smartphone, lol then download TomTom and just add a GPS receiver...
Whats a GPS Receiver
The GPS Receiver is what gets the signal from the satalites to triangulate your position, it can then plot this on the maps and track where you. There are other devices which sometimes are advertised as sat nav that dont use GPS receivers and dont have one built in these are basically just glorified road maps, which you have to tell where you are and they just give you directions, true sat nav tracks where you are and plots the route from your current location to your destination... Some PDAs or other units have GPS receivers built in like Toys R US do a cheap one, but these use cheap navigation software which often doesnt get updated. With a normal PDA and TomTom you can use a Bluetooth GPS Receiver usually about £50.
TomTom
I would recommend tomtom as opposed to others as I have found this to be the best user interface, I built a full carputer for my car which ran a laptop from a front touch screen, the sat nav that I had which was about 1 gig in size was absolutely rubbish compared to the simple tomtom running on my phone...
TomTom has great support with hundreds of sites to solve any problems and the extras you can download for it, the maps are usually more upto date than other software I have tried... Just the whole thing is just generally really easy to use with a clean 3D mapping system.
TomTom has a POI (Points of Interest) Database system where you can add your own POI such as a database of Camera locations, or locations of major shops like Asda... Its the best all round package in my opinion
If you are a professional driver I would recommend the all in one unit because you can leave this in your car otherwise if your only using it for everyday journeys and occasional outings then the PDA or Phone version, gives you the same options but in a device you can use for other purposes...