Some more technical details on the Gameboy Advance:
CPU: ARM7TDMI running at 16.78MHz (an integrated Z80 is included for Gameboy compatibility, but it can't operate at the same time as the ARM)
RAM: 32KB internal and 256KB external RAM (differ in access speed only), 96KB VRAM, 1KB Palette RAM, 1KB Object Attribute RAM
ROM: 16KB system ROM (BIOS)
Gamepak (cartridge) size: up to 32MB ROM and up to 64KB SRAM
Display: 240x160 dots at a maximum of 32768 simultaneous colors, not backlit (requires a light source)
Image system: hardware support for scaling/rotating, alpha-blending, fading, and mosaic effect. Both tile-based and bitmap-based video modes. Up to 128 independent sprites.
Game controller: Directional Pad and 6 Buttons (A, B, L, R, START and SELECT)
Sound: 4 analog sound chip channels (Gameboy compatible) and 2 PCM channels (8bit), headphone plug
Communication: 1 serial communication port for multiplayer gaming
Power supply: 2 AA-type batteries required, no AC-adapter plug available.
If you're interested in Gameboy Advance Development, you might want to check out http://www.gbadev.org and http://www.devrs.com/gba/