The hardest part will be getting the 6.5" screen. There are 6.5" LCD panels but no controllers available in single quantities. Most people go with 7" and either have the DVD drive elsewhere or no DVD (use USB thumb drive or something instead). I am putting a USB DVD in the glove box.
Or you can go for the Double Din screen w/ built in slot DVD that will be coming out for sale soon. There is a big thread about it here, just search for Double Din.
There are plenty of motherboards that will fit in the dash behind the screen. There is a sticky that lists some good ones. Then get a small drive, XM adapter via serial or USB, AM/FM radio with w/ 4 channel amp via USB, GPS module via USB or serial. Cram it all behind your screen w/ cooling and power supply.
You will also need to carefully choose software for your front-end and other uses.
Easier said than done, of course...but all of the info is available on these forums by searching.