To achieve this, nVidia renders the scenes in a frame buffer that is four times the size of a regular frame buffer. Each frame is then filtered down to the default resolution, which achieves the smoothing effect on the edges.
Intermediate steps are also possible.
Find out more about FSAA in Tom's article . At 800x600 they are at the same low level as at 1600x1200. This is not surprising, because the chip must render the scene internally at 1600x1200, and then filter it down to 800x600. The explanation is very simple. Personally, I am not too fond of FSAA in a shooting game like Quake.