I don't really want to change the Gameboy too much from its original (on the outside at least :-) ) but I would like that PC/104 i've been eyeing as a potential candidate for use to fit and work out. If anything more research will bring more answers as where to proceed and opening up the gameboy will provide those answers.
For a second lets show what the GameBoy specs are from under the hood:
Custom 8-bit Sharp Z80 at 4.194304 MHz (has a slightly different instruction set than a standard Z80, and integrated sound generation)
8 kByte internal S-RAM
8 kByte internal
256 kbit, 512 kbit, 1 Mbit, 2 Mbit and 4 Mbit and 8 Mbit cartridges
4 channel stereo sound. The unit only has one speaker, but headphones provide stereo sound
Reflective LCD 160 x 144 pixels
66 mm (2.6 in) diagonal
4 shades of "gray" (green to black)
Up to 4 Game Boys can be linked together via serial ports
6 V, 0.7 W (4 AA batteries provide ~35 hours)
90mm(W) x 148mm(H) x 32mm(D)/3.5 x 5.8 x 1.3 (inch)
(Courtesy of Wikipedia)
It will be cool pushing this thing from a 4Mhz Z80 processor to a 300Mhz+ ARM processor. The sound will definitely have to go, and with a little research, the power source will come into question (6V might be enough but I am not an EE to really know).