More GPU memory means you can load more graphics. First u should check supportings that is OpenGL, DirectX, ShaderModel. Then u should check performance and performance depends upon core-clock, shader-clock, memory-clock, bandwidht, GDDR Type etc. etc.. Then check the memory because memory gives performance of only 6%. Now-a-days 1gb memory should be minimum for games, 2gb Avg and 3gb maximum.