I have been wanting to build a car PC for the longest time. I hate having to get out a map and try to figure out how to get from point A to point B, I would much rather prefer to type in my location and then my destination and have a computer instantly compute the fastest/shortest/ most economical route. Or instead of punching in a starting location just have it there for me and then tell me where to turn. Pretty much cut out all the BSA stuff I have learned. 

Whats in My Rig:

•     Homemade Gateway M275 Tablet PC
  • Centrino  1.6GHz
  • 512MB PNY DDR333
  • 60GB 4300RPM or 7200RPM (haven't decided yet) Hitachi 2.5" HD
  • 14.1" Tablet LCD
  • DVD/CDRW Drive
  • Internal WiFi b Yeah I know that sucks, but i got it covered with a USB dongle and a parabolic AL bowl!
    
•  Sima 325 watt power inverter (no audio noise yay!!!)
• 10" Alpine Type S
• 250 Watt Kenwood amp
• LiteON slim external USB slot load DVD burner
• Zonet 4 port USB 2.0 hub
• Microsoft GPS receiver
• 4 neon tubes, located under dash and bucket seats
• JVC HD Receiver
• 8.4" Lilliput touchscreen
• Custom LED light job to most in car lights
• Custom LED switch panel
• Custom Netgear USB WiFi b/g satellite dish
• Dell bluetooth keyboard/mouse/dongle
• M275 laptop port replicator 

All together it cost around $1000 for all my car upgrades, but about a third of that was covered by my loving family and friends over the last few years.  The rest gradually came out of my pocket during that time period.  $1000 isn't to bad when you consider their is a fully functional laptop thrown in there.   For more information on what I paid you can check out a very detailed spreadsheet here.