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
BMW 7 Series
BMW 5 Series
BMW X5 Series
BMW 3 Series - Great informational sites -