USB flash drives, portable USB/FireWire hard drives, and WiFi are a few of the ways to get data onto and off of your car PC.
One of the things you should know before you get an in-car computer is that you are going to have to feed it. Car computers are multimediaivorous, subsisting primarily on digital audio, video, and GPS coordinates for roughage.
Seriously, though, you are going to want to get your media into the car, and probably update it from time to time. Some people prefer to perform a onetime dump of their entire MP3 collections into their cars and leave it at that. Others want to be able to listen to daily music feeds from Podcasts (http://en.wikipedia.org/wiki/Podcasting), books on tape (http://www.audible.com), or their own email and documents converted to MP3 using computer text-to-speech synthesis (http://www.nextup.com).
Although wireless is sometimes touted as the holy grail of car-computer synchronization, 802.11a/b/g may be too slow for routine large media transfers. Also, any WiFi solution depends on your car being parked close enough to your home network to get a decent signal and requires your car PC to run when you aren't in the car, which can potentially drain your car battery.
Table 6-1 compares various ways to transfer data to your computer. Which method you choose comes down to how much data you have to transfer.
Table 6-1. Network transfer speeds
Theoretical speed (in Mbps)
Maximum (in Mbps)
Minimum (in ...