yea, i did the same thing as you one night as i was bored.. been thinking of a car pc for about a year now or so (ever since via released the epia-m mini-itx boards and i bought one.. )..
so here's what i want (similar to you) with my project:
-lcd monitor mounted on dash somehow
-easy method of input to pc (ie touchscreen/custom keypad/etc)
-no major electrical mods to car (since car is an accord 2002, don't want to mess with warranty too much)
-not worried about having system turn on when car does, wiling to have manual power on for pc
-very small form factor and accessible by driver (again to power on pc and put in dvd/cd)
-gps optional but be able to do as an easy addon
-mp3 playback through existing car audio system (i already have a very nice headunit (pioneer DHP-7500MP, w/mp3 player and amp and sub)
i am currently working on a project myself, just started this week.
i've toyed with the idea of a micro-atx based home pc in my car but decided that the power issue was annnoying and all... so i scratched that idea... too much electrical which i do not know yet.. plus, i don't need that kind of raw power..
so i've decided with this:
-using a minibook pc (1.3ghz celeron) w/dvd drive (from tigerdirect, $449)
-Lilliput 7" Touchscreen VGA-LCD (has two rca inputs, vga input, usb touchscreen, ac/dc and dc/dc power cables, again so i don't have ot rewire or anything)
-custom mounting bracket from ebay, really sweet one too! ($9)
-3 port cigarette power adapter (w/built inline fuse)
-car dc laptop adapter for power to minibook (depends though on how much power i need and what voltage i need)
i think i'm gonna mount this baby on my dash, gonna probably remove my clock (and if possible, might see if there are any hacks that let me use my clock power to power lcd if its not too hard) and have lcd mount mounted in that spot and then run wires in dash (hidden)
thinking of putting the minibook pc in my armrest area but need to think about heat, so might need to put in cooling of some type..
as for O/S, i plan on using Windows XP Prof or perhaps Windows 2000 Pro, but gonna use desktop x for my shell (already create my own theme for use in car so i have large icons on desktop for winamp and powerdvd and it looks sweet!!!)
honestly, i believe that a 1.0ghz would suffice for what your doing, not like your gaming in your car... i'll post updates if your interested..
just what i'm doing , perhaps will give you some ideas or not..