My understanding is that the AGP Aperture is used for graphics textures and is only used when your video card runs out of memory. You're using a 6600GT, so you must have either 128 or 256 on board. You also have 1Gig of Kingston. Quite frankly, you may as well set your Aperture as high as you can, coz it will hardly ever be accessed if you have 256 on your 6600GT, or if you've only got 128 on board, then "borrowing" 256 out of your 1Gig is gonna make sod all difference to the speed at which your system runs games anyway.
Happy Gaming.