my suggestion would be:::: 'be patient' it's taken me almost a year to build my car-pc due to lack of coin, during that year I wasted a lot of money on cheap "this might do" alternatives, but now that I have the full pc based system up and running, i'm glad I gave up on the "cheap alternatives"
by being poor,,, therefore being forced to be patient, it's amazing what you can learn by spending 10 or so months scouring these forums,, once you actually get round to building your system, you've got a pretty good idea on what your doing.
out of the 500 bucks your thinking of spending on a "this'll do for the time being" you can get a long way on a "this'll do for good" project
for example: here's what I've spent so far:::
((((AUSSIE DOLLARS))))
Misc crap--wire, relays, psu, etc..
---$100
Epia M-10000 m/board:
---$220 (ebay, used)
512 Ram
---$80 (new)
Power supply
---$100 (300 watt) (goin' the inverter, no ground loops, noise, etc.. thanks to "scouring the forums" also, ignition based startup-shutdown control, once again due to "scouring the forums"
Hdd 80gig (3.5")
---$50 (ebay, used)
VGA touchscreen
---$250 (
ebay, new)
GPS
---$150 (new)
total=== AU$950 USD$720
as you can see,,,with what I,ve spent, It's pretty close to the 500 your thinking of spending on a project your not gonna be happy with,,chill for a bit, you'll be glad you did
Edit-- OK I forgot the dvd bit---come-on how cheap are DVD-ROM's now??? add AU$50