I took quite a while before I decided which one I wanted. The prices varied dramatically but in the end I decided on a Navman icn 635 and I am most impressed with it. It comes with a 512mb card and the prices I saw before and since were anywhere from £279-£599. The £279 was in Dixons. It has got a large limpet to stick it on the windscreen which can easily be removed to avoid leaving it for the crooks to smash the glass!
It has a full coverage of the UK and main roads on the continent. Accuracy is above average I feel.