You can Retrofit your BMW with a full Navigation system, although it is more challenging than just swapping out your display.
I have some of these parts on the following webpages, but do not have widescreen displays for the 3 series or the misc brackets detailed in these part numbers. I also do not have kits that come with all of these parts.
Bimmernav.com offers for sale at very competitive prices parts for your BMW navigation retrofits. See online webstore for details
There are a number of websites that detail this procedure and documents from BMW that detail this effort.
Great links of information can be found here
Yahoo Groups BMW Nav Owners Forum
Great Webpage on retrofitting Navigation Systems in BMWs
Detailed 5 series retrofit page with part numbers and photos
Navigation O/S software versions and how to upgrade
BMW 7 Series
BMW E38 Installation instructions
Virgin 740 Installation in a 1995 BMW 740i
Parts Required for a 740 Virgin Install
www.E38.org Navigation information page
BMW 5 Series
Tencorners 5 series Full retrofit
Parts Required for a 540 Virgin Install
Another BMW Published retrofit Guide for 5 Series e39 nav retrofit instructions (non-touring)
BMW X5 Series
BMW 3 Series - Great informational sites -