If I were going to make a "car" case, the first thing I'd suggest, is decide what kinda vibe you're going for. Do you want some old Model T Hotrod vibe, Muscle Cars (GTOs, Barracudas, etc), or something maybe a little more "cruise" worthy.
After that, do some research on some of those cars. For instance, if you wanted to do, say ... a cruiser, and were going for an old Mercury or Bel Air, something like that. Then look at the parts that stick out, you can find TONS of photos and get more at local swap meets and on the internet. Also, you might incorporate actual parts, aftermarket chrome bits, etc.
When I think of this mod, I think of my grandad's old car, it had this huge chromed dash, with gadgets EVERYWHERE, and that's just begging to be the front of a computer case. Depending really on the "vibe" you're going for, is what parts you'll want to focus on really, if you're going for the suped up hot rod/muscle car for instance you could make a tacometer for the case that displays temp inside the case.
Anyhow, just a couple of ideas I thought I'd throw at you. Good luck with it, it has a lot of potential.