I actually wondered the same myself. Lately, everyone has been recommending 2GB for builds instead of 1GB. So, it got me curious to see how much a difference in performance there would be with 2GB vs 1GB.
Well, overall the extra GB didn't really make a huge difference. During games, there was practically no difference whatsoever. I have read benchmarks which support this as well.
Where I did notice a difference (a very small one) was when I go to open or close programs. There is a little more responsiveness.
However, would I consider this to be a justified purchase? Not really.
If I were to turn back time and had my choice of upgrades, I probably would have gotten another hard drive or something