There is a choice of how the 256M of RAM is divided between the ARM and GPU:
arm224_start.elf : 224M ARM, 32M GPU split : Maximum ARM memory. Good for ARM desktop use. No accelerated video or 3D possible.
arm192_start.elf : 192M ARM, 64M GPU split : Reasonable ARM memory. Simple video (omxplayer) or 3D (quake) is possible. This is the default.
arm128_start.elf : 128M ARM, 128M GPU split : Use this for heavy 3D work, or 3D plus video. Needed for XBMC.