Since factory nav retrofit is too costly, I went with a portable unit the garmin c320, due to it's price and size (need to detach when parked in NYC). Got it on sale for 450 from amazon. This unit comes with 128 mg SD card instead of a 2.2 gig harddrive like the c330. I can fit the entire state of NJ and NY on there which is really all I needed.
Installation was very straight forward, Nav interface was great, reception was great, and logic is almost dead on accurate so far. It performs better than the nav in my roommate 2004 330cic. Very happy with it overall however the screen is a bit small when you mount it on the windfield. and the look is not all that clean with the power wire hanging down.
Highly recommended if you are looking for a straightforward portable GPS unit.