There's a lot more to GPUs than memory. The main spec are the stream processors. The more, the better. For example: a 1gb 9800gt (112 processors) is worse than a 768mb GTX460 (336 processors). The only thing memory helps for is gaming at bigger resolutions, I believe (correct me if I'm wrong).
Also, the type of memory is really important (from worst to best: DDR2, GDDR3, GDDR5).