Hi there guys,
Finally finished it (roughly ). After hard work and being a real noob, I got everything fited. The hardest part was passing the power cable from the engine compartment inside the car.

Specs:
Dell Optiplex SX270, Pentium 4 2.8GHz, 512MB, 80GB HDD, Windows XP Pro
Lilliput 7" Touchscreen

Anyway here it is:
The screen


The PC and the Amp


Unfortunatly it was on for only 1minute before it died out (which I will investigate tomorrow, might be the fuse in the DC Regulator.).

But there it is. I didn't go for the control panel mod as it seemed too much work and I didn't want to get rid of the HU.
Anyway cheers to those who helped me out.