If I were playing a game (lets say Unreal Tournament 2004 demo), and I had AA and AF turned off, then I went and turned them on at 8x and 4x, what would be some of the major differences that I would see?
You got to balance it. Like if you're running 1600x1200 without any AA or AF, you will probably get the same frame rate as running 1024x768 with 4x AA and 8x AF. However, the 1024x768 with 4x AA and 8x AF might look better because of the diminished jagged lines. You just got to experiment.
I usually try to keep my resolution at a certain rate, and if i get more then 60 fps in a game, I increase either the AA or the AF depending on what time of game it is so that I get the best possible image quality and I still have a fun experience.