You wouldn't be the first person to do that.
Three things to consider:
Power - Your car only has so much of it.
Heat - A computer can only take so much of it.
Space - You've only got so mcuh
Remember that high-power PCs also gobble up electricity and generate heat. Realistically, there's no need for a high-power PC in your ride. You're not going to be doing 3-D rendering or playing gamez or dominating the SETI@Home stats.
Build a PC that will do what you need that will fit the parameters you have for power, heat and space, and you're doing good.