The most well known portable gaming device in the world is Nintendo's Gameboy. Originally released in the late 80's it has sold more than any other portable game console in the world. Several years later Nintendo released the "color" version of the all time classic!
Specs (of the Gameboy Color model):
Processor: Z80 Sharp processor
Processor Speed: 8 MHz
RAM: 32K
Video RAM (VRAM): 16K
Colors Available: 32,000
Colors On Screen: 10, 32, or 56
Resolution: 160x144 @56 colors and 320x288 @24colors
Sound: 4-channel FM sound
Controls: 8-directional D-Pad, A, B, select, and Start buttons
Power: 10+ hours on 2 AA batteries. AC adapter separately available