The GBA has an (un-cached) 16.8 Mhz ARM7 TDMI CPU, 32kb of internal RAM, 256kb of external RAM, and up to 32Mb of ROM (dependent on the game). Apart from this, it has a quite powerfull 2D-engine, with support for up to 128 sprites (without multiplexing - more if multiplexing is done), tiled or linear backgrounds, and some blending. It also has a really cool hblank-triggered DMA, allowing SNES mode-7 style effects.