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

BimmerBoard Navigation Forum

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

Widescreen nav display

BMW Navigation Software FAQ

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

X5 World Website