1) It's good in that it allows more detailed high resolution graphics than a card could normally handle.
2) This comes at a price of lost FPS in high resolution.
This is why the 8800GTS 640mb is about the same as the 320mb in most cases but moves ahead in some games in high resolution.
Anytime the GPU needs to swap items into general ram you will start losing performance quite simply. However, I would not lose sleep over your card. Benchmarks show that it's rarely an issue with your card.