you wont HAVE to upgrade your car PC to win8. Win7 will be viable for far longer than your hardware. As i understand it, win8 will cold boot faster, so if that's important to you then it might be worth it.
In my vehicle i'll stick with 7. .Net remains supported, 7 is "service pack stable", meaning they're not going to do anything radically different with the interface like i've heard rumors with in 8. Another reason i'll stick with 7 is because car PC hardware isn't ready for half of what 8 delivers. The metro interface is crippled without a multi-touch screen, and unless you're using the surface acoustic Lilliput, that's what you're currently stuck with.