NOTE: Earlier versions of RetroPie included a default EmulationStation theme with large image files, that would eventually cause a 'white screen' when too many systems were active. This 'white screen' bug has been fixed as of RetroPie 4.2, and since emulation itself uses very little video memory: there is no benefit to raising the video memory split.
Also note that in order to ensure sensible memory splits across Pi models, RetroPie utilises the gpu_mem_256, gpu_mem_512 and gpu_mem_1024 overrides, which apply to Pis with that amount of memory (for example, the Pi 2 has 1024MB memory, so will use the gpu_mem_1024 setting). This setting overrides the gpu_mem setting that is described below, so if you still want to adjust the memory split, you will have to manually edit /boot/config.txt and adjust the relevant value, or delete the lines entirely.
A complex display of the GPU memory usage can be viewed with:
sudo vcdbg reloc
Since the raspberry pi is a SoC the CPU and GPU share the same amount of RAM. The following option allows you to choose how much RAM you allocate to the GPU compared to the CPU.
You can choose a split from 16/32/64/128/256 (the settings may be different depending on the version of pi you have)