I just "finished" my Car PC installation (will post pics soon).
Car:
2000 Ford Mustang GT Convertible w/ 68k miles (bought about a month ago). ($4000

)
New paint and minor repairs. (~$1200)
Computer:
Intel Atom 330 dual core 1.6Ghz on a D945GCLF2D mobo. ($79)
M2ATX power supply. ($50)
2 GB DDR2 667 Corsair Value Select RAM. ($20)
16 GB Super Talent MLC SSD. ($74)
Airlink 150N wireless USB adapter. ($15, but I already had it)
Comp USA bluetooth adapter. ($2, but I already had it)
4 port USB HUB. ($6)
GlobalSat BU-353 GPS reciever. ($30)
ADS Instant FM Music usb adapter. ($20)
Xenarc 7" touch screen 705TSV-B. ($200)
30GB iPod Video. (have had for years... not going to include in price)
Palmer Performance OBD-II to USB cable. (FREE! for me)
palmerperformance.com
Audio:
4 channel plus sub Alpine amplifier. (Free, got it from a cousin a few years ago)
8" JL Audio Sub. (Free, also from the same cousin at the same time)
Misc:
Mounting hardware. (~$50, but I didn't end up using all of it)
Replacement console insert so I didn't have to cut mine. ($20)
Total cost:
$5,766 including car and stuff I didn't use.
$496 for just the PC.
Installation:
The goal was a little ambitious and uncompromising. I wanted to create an OEM looking final project which was competely reversible (able to return to stock look easily).
I'll include lots of pics and video later, but for now here is a bunch of text...
The stock 2000 Mustang has a top HVAC control and 2 single din spaces below with the lower being recessed several inches. I relocated the HVAC controls to the lower din space and hack sawed the plastic seperator between the two upper spaces creating essentially a double din space (for the screen).
I created a mounting bracked for the Mobo using 22 gauge steel covered with 1/8 inch plexiglass covered with mounting tape for insulation and shock absorbtion. The M2ATX was mounted in similar fashion, minus the steel plate. The SSD was mounted on the bottom of the steel plate using part of a 2.5 inch drive enclosure and mounted with mounting tape. The total PC fits into a single din spacing behind the monitor with the power supply mounted where the HVAC controls used to be (also behind the screen).
I then dremeled (there was lots of dremeling and hack sawing) spaces into the console piece for an external USB port and Aux input and secured them to the console piece with "mighty putty". These ports are located just above the new HVAC controls and are invisible unless you're looking for them.