Well, I've done the same... 640x480 and it's mostly readbale on my 28" Tv - What It'll be like on a smaller screen, I dunno yet...
I am running Microsoft Accesibility Magnifier! So if anything is too hard to read (namely road names I've been testing out on!) I can hover over it and I have a small bar at the top which magnifies - can certainly read it then! I could live with this.. the output is very good for Composite!
I'll let ya know how I get on finding a decent screen for the car.. STN 7.2" is looking possible.. but, I'm a little worried about these trails.. I'll be watching DVD's.. but, it won't be cold.. cus' it's above the vents..