Which graphics card are you talking about here?
My advice is to go for the second card, it provides more bang for your buck and at $109.99 ($89.99 with rebate), it's a good price.
More graphics RAM helps it handle more textures (surfaces) in games without loading more data. It's not always the case, but within the same series (i.e. two 8600's) more RAM is better.
Does that help?