depends on the resolution... the Video Ram only makes a difference in two ways:-
1 - min frame rates.
2 - being able to play on higher resolutions.
u need about 1 GB to play on 1980x1080, which is Full HD, but it think a 512 MB will still run, though on an inferior level.
but up to 1680x1080, 512 MB and 1 GB cards wont make a difference on the max frame rate or the average, only on the minimum.
1 GB ram is always preferred, however, 1gb ddr2 128 bit vs. 512mb ddr3 256 bit, id choose 512mb ddr3 256 bit.
Specific graphics cards from each would be more helpful, though.
Tc.