in fact, you haven't read much.
The FAQ http://www.mp3car.com/vbulletin/the-faq-emporium/
okay to answer you.
Most carPc installed use 7" screen and some 8". And software/front end like RR are written/skin with a mall monitor (7") in mind. So they are okay.
There is NO need to buy new PC if your old one capable of what you need for carPC (GPS, Music, movie ..... Porn). Take a look at mp3car's store to see the power option (Power Supply) and build your carPC around that limitation. Currently the most powerfull PSu is DSATX which is 220W but not widely available as it is in beta stage. Or you can build one like mine (Sproggy 4.5) which is the most powerfull PSU, but PITA to get parts.
RR has it own subforum, go there and read the faq, but basically if you know how to use winamp, you can use RoadRunner.