Seeing as we are all throwing our assholes opinions in here, I will throw mine in.
One of the issues with air cooling is, you bring all sorts of ****e into your PC case and around your components, especially if you hook up a fan to draw air in through the 'baffle' (I know the suggesting was to blow air out of this but then you aren't really bringing cool air in. Water cooled (as long as the case is air tight) will prevent this, but then you need to water cool the video chip, memory and HD's (is there water cooling for the drives?)
Bringing cool ar in from the cabin is probably the best idea, like someone said, you ain't gonna wanna be in a hot enviroment yourself. But rigging this up could be messy and/or ineffective.
Plenty of heat sinks and somehow contact with the vehicle body to help as an extra heat sink, but then seeing as how hot the vehicle body can get on really hot days, maybe not.
I think each individual has to experiment and finds out what works for them. Also your location in relation to the equator wil be a factor as well. (I reckon the canucks are laughing at the moment, saying "what heat issues?" In fact they are probably asking "How can we prewarm our PC's before starting the car?")