RyRy, there are a plethora of automotive grade portable navigation units on the market; it really depends on what features you will require.
I have done a multitude of research on these units, and in my opinion, the Garmin 2620/2720 is the best all around unit. It has it pros and cons, like all other units, but after 3 months, I am still 100% satisfied in my choice. It has some features that no other units in its class have, and the ergonomics of the menus just make great sense. To me, it's the Honda of the bunch.
Please note, screen size was probably the most 'sought after' feature for most people when deciding on a portable unit. I have found time and time again, people were relieved to have gotten the unit with the smaller screen. It seems at first glance, the unit with the biggest and most colorful LCD would be the best choice, but you will find out that once installed, you very rarely will be looking at the unit itself. Those eye candy features wear off really quick, when lugging around a big unit, or taking up a lot of dashboard space, or annoying you when seeing a big screen out of the corner of your eye when driving at night.