Just a further point,when you use higher res and bump the AA up it will use more video ram,games are getting more demanding all the time ,Doom3 and HL2 will be two good examples and I won`t bother meantioning current games, so the more ram that`s free(set a side in AGP aperture size) for graphic textures (if the AGP card needs to borrow it from system ram)etc the better,remember at one time 64mb was enough now 128mb is the common standard , 256mb will be the common size down the road.
Bottom line it`s depends on the game,resolution,AA etc you use.Right now you can get away with small AGP size if you`ve a 128mb card,down the road that`ll change .