4gb is enough for most games in win7. The reason you will see a performance loss is because of dx10. Win xp was dx9 only and it doesn't require as much processing. dx 10 requires more gpu processing due to some new features, but it doesnt really help image quality much. If you right click on the shortcut to your game and select "run in dx9 mode" the game will run better with no real loss in image quality.