the amount of gpu ram is only a factor if it is not enough. for such a slow card (630) even 8gb of memory will not help. even a 560ti 1gb will blow that 630 out of the water.
get the faster card, memory is not everything.
bf3 eats up around 1.7gb of gpu memory on ultra @1080p. just to cite an example, but of course don't expect ultra (using bf3 as example) if you're using a low end card even if it has 4gb. even the 650 wont be able to handle ultra