Benny, I'm in the same boat, about to install a MM into my car. Infact I have all the components required and have bench tested the setup already, all good. I've played with all the front ends and would have to say that NeonBoomBox is the most promising at the moment. Qcar is great, but costs a lot even if you don't want the navigation portion of the app. AMP is still pretty good, and FrontRow (built into your mac) is also rather good, probably perfect if you have a rotary controller which I do not, yet.
VGA splitter would be the go for multiple monitors, I do not know how this will impact the quality or brightness of the displays however. I'd also like to look at this for the future so I can put a screen in the back of the car for my soon to be born daughter
(wiggles anyone?)
I bought a 1.4Ghz PPC mac mini with a gig of ram for about $240 USD, it runs very well and all front-ends I've thrown at it work nicely. A carnetix P1900 PSU and macpac cable and a cheap 7" lilliput screen. I've spent about $600 USD in total so far. The only thing I really need to do is install this gear into my car and make a custom mount/facia for my screen.
all good fun