Noooo more memory doesn't mean better. In fact, if a card has more RAM than it can use efficiently, it will be worse than the same card with less memory. I believe the 6600GT is an example of this. The 6800GT with 512mb is probably better than the 256mb since it's a pretty powerful card, and can probably make use of all that memory. But that doesn't make it better than a 7800GT. The 7800GT is way better.
