The CPU and GPU on the SoC used on Raspberry Pi share the system memory. The amount of memory for the GPU can be adjusted in the config.txt file with the gpu_mem option. If you are not using any software the needs the GPU you can maximize the system memory by setting this to 16. In case you are using software (OpenGL ES) or hardware that needs the GPU you should increase it or use the default of 64.
The default value is 64. Minimum value is 16; maximum value is 192, 448, or 944, depending on whether you are using a 256M, 512MB, or 1024MB Pi.