OK, so 2gb vs 1gb. This is the amount of memory they have. So your RAM is 8 or 16gb. More is better in this case. Remember that a card with 2gb of memory will play smoother than two cards with 1gb each. A good example of more is better is the gtx titan vs 2 980s because the titan has more memory on a single card meaning it doesn't need to waste time transferring data from both cards before sending the input signal.
Now as for your choice of graphics card, I have to recommend the 750ti or 960. Its an amazing card with choice to overclock. I've played with itthe 750ti and its brilliant for older games but for newer ones check the 960. Search on youtube for videos on 960 fps test gta 5. I have to place nvidia over amd because the latest second tier card, the 970, from nvidia smashes the 290x, from amd. And if the price puts you of, remember when it comes to pc parts, you almost always pay for quality.