GPU can only display as much as it's told to , it's told what to display by the CPU , cpu gets it's instructions from Memory , and memory is loaded from disk ...
to slow a disk and the memory waits for the information to load up before it can start sending the data to the cpu , too little memory and the cpu waits for the instructions , to slow cpu and the gpu waits for the the cpu to catch up.
so it's allways better to have your graphix card "slightly" ahead of the cpu , but the cpu does so much more than just graphix ( like emulating etc.. ) and because disk is so slow it never never hurts to have to much memory ( is their such a thing? )
for most PC games it's the GPU that is lacking on alot of systems ... but in the case of emulation , their is so much work the CPU needs to do for each frame , then cpu would be the first thing i would look into.
memory is like crack for the computer .. it allways wants more. ( if i could afford 2 1G sticks , i would )