2x and 4x Full Scene Anti Aliasing uses a method called "super sampling". QUINCUNX uses multi-sampling. It provides FSAA that falls somewhere between 2x and 4x FSAA without the performance hit...however, if you ever sit and play with them, you will notice that QUINCUNX makes textures blurry. Depending on the game, this can be slightly noticeable to totally unbearable.
