For an ammo box? I'm sure you could get away with something decently powerful. It would definitely need to be small form factor and with no air-flow built in, you would have to spring for liquid cooling. Modifications will have to be made to it too for the IO and power button. I'm sure you can fit something pretty good in it if you jerk stuff around inside like having your graphics card mounted parallel to the motherboard rather than perpendicular. As for power, you can get away with quite a lot, especially if you're using a non-standard configuration. Width wouldn't be an issue since your graphics card won't be poking out so much.