Weak dual-core cpu android phones can handle the android port of the reicast emulator without any problems.If you ever have something that big on your writst, expect it to deflect bullets, or something like armor.
Captian america's sheild or some other secret agent weapon. I rather have something like a GBA sp, with a folding lid and buttons attached to the screen, but one of those open source emulation hand held like GCW Zero, had interested me until nvidia shield came out
Maybe mod a raspberry pi to be mobile device, it should be powerful enough for anything before Dreamcast