ut is a good example of AA use.
the game is very cpu sensitive...the actuall graphics arent a problem at all since its only dx7/8 level, but the physics etc what the cpu has to deal with brings fps well under 100 specially in onslaught.
so ur graphics maybe capable of say 150FPS rendering the graphics, but ur cpu may only be able to push the calculations at say 60FPS, so basically u have 90FPS that u just aint gonna see, so u can put on a little AA and AF, this will bring down the graphics FPS to say 100FPS, but ur cpu is still only doing 60FPS, therefore u dont see any performance difference, and it looks better....so we can effectively call it FREE
as graphics cards produce square pixels, when u make a diagnol line out of these as u can imagine they have a jagged edge, AA seeks to remove these edges by multisampling the final pixels to smoothe them out