Which isn't true, really.
More importantly you want to look for memory bandwidth. Graphical intensive stuff like shading, lighting or anti-aliasing take up memory, the higher the memory bandwidth, the more amount of this stuff you can do at a time. Having the extra vram is only good as a buffer so the gpu doesn't have to relegate itself to much slower system ram or hdd memory if it has to.
High memory bandwidth is achieved through use of quad data rate memory (gddr5) and high bus widths, most importantly.