As I said, it depends on the card. If you have a high end card like GTX 680, 1 GB vs 2 GB will feel a lot. However, if you have something like GTX 550 Ti, you will never tell a difference.
I just saw this after I made my last post (you posted before I finished my post :P). That's a lot closer to what I'm looking for. Now just another question so I don't have to start a whole new topic and this is somewhat related. If I get the 1gb overclock version can I use it in sli mode with the 2gb even though they have a difference in memory and clocking speeds?
I do realize that having two of the same card is how it works. this is just more of a food-for-thought-type inquisition, theoretical.