I've been using car pc's since 99 and they have come a long way. Having said that, the integration with the free solutions out there are good. I chose one of the pay solutions (Centrafuse and StreetDeck) because I wanted support and a polished product. I've played with the stock nav system in the G and it's very nice. If you are going with the aftermarket carPC solution the Infill G4, the Azentek, or the CT650 from Carputer Technologies will give you the most finished looking solution.
Also note that the computer system (Azentek or Infill) will cost you about 2k-2.5k (so the offset of not buying the factory nav). With those you won't need an external amp as they are drop in solutions but with extras you will more than likely be up to about 2.7-3.0k when complete.
The CT650 route will end up about the same, but you have the screen unit separate from the PC and depending on how much horsepower you want for the computer you will end up about the same (if you get top of the line equipment) and I'd expect the price to be around the 2.5k mark.
Granted, this is going with the best/fastest stuff available; you could get away with a cheaper solution. The last issue is the double din mount for the G. I know the older ones had to use the double din from the Japanese market since there wasn't one offered here, so I can't say with certainty what's available for the 08 model at this time.