The AGP aperture allows the graphics card to call on free system RAM for texturing, allowing the card to get on with the rendering.
To say set it at double the card size is wrong. What if I'm operating 64MB RAM and 32 MB card. Probably will work, but it might cause problems.
I've always been told to set it at half the main RAM. So 128MB of RAM = 64MB aperture.
"Now drop your weapons or I'll kill him with this deadly jelly baby." :wink: