If for factory gps, there was a post several days ago about this. Conclusion: the retrofit is not worth it. If aftermarket, from the cheapest to the most expensive:
A) ipaq (pda) and gps antenna (delorme) plus gps software for.
This is the setup i am using. So far, works like charm. If i have extra dough ($$$) after this holiday season, i want to go option C or D.
B) aftermarket gps unit, like garmin 2160. Saw one in best buy. Looks cool and stealthy. Pricey($899?)
C) pioneer/kenwood/sony lcd plus gps controller box and antena. The software will be propietary for which ever brand of gps unit you are getting. Around $1500 - $2000 i think. Very expensive to upgrade software/maps as the software is proprietary. Also will be hard to find location to mount the lcd, i dont think you can just replace the factory lcd (function)...maybe i am wrong.
D) create a car puter = car computer. Basically an lcd, mini computer using an DC inverter, and gps software. Again, hard to find a location to mount lcd but really easy to find cheap upgradeable software. This thing is really hot in car mods these days. Do a search in google, you will see what i mean.
I use a Garmin GPSMap 76s, which has a nice large screen,
it has a lot of great features, which are the same as the GPS system in the car (not color though) (the only thing I don't know of is if the MO version will plot a "street" route, rather then a "as the crow flies" like the garmin does)
The 76S , has uploadable maps, (city details, waterways, topo, etc.. and has better data then the MO wouldh have)
I use mine all the time.. the only "problem" is the security issue..
I have to hide it everytime I park the car.. (damn thiefs..)