I did it the el-cheapo route in my X5. I used video-out and audio-out on a spare laptop and routed that into the TVonNav module inputs (via my DVD player's extra inputs). So now, my passenger can surf the web and all passengers can see the screens, listen to internet streaming audio, listen to MP3's, pull up live weather forecasts, etc. My BT phone handles the internet access. I can control the laptop with an RF mouse if I am alone but I have no desire to to surf while driving!! I'll search around and see if the X3 uses the same components for nav, tv module, etc. Also check with NavTV.com and TVonNav.com and see what they say about the X3. All you need is an overlay device to get the image on the screen, then you can route any video signal through it. (PC, DVD, camcorder, etc.)
Here's a screen shot of our clubhouse on the nav screen:
