According to 'The Definitive BIOS Optimisation Guide' v.6 at Adrian's Rojak Pot:
The AGP Aperture Size is set by the formula: Maximum usable AGP memory size x 2 plus 12MB.
What this means is that if we really want to get into optimal AGP settings then we need first to ascertain how much usable AGP memory we need, then double it and add 12 MB on top.
The reason why the usable AGP memory is doubled is that there needs to be an equal amount of write combining area. Add to this 12MB for virtual addressing and there we have our AGP Aperture. Of the 64MB that you may set aside as the Aperture, the actual usable AGP memory size is only 26MB!
Of course, this memory is not used unless the system requires it and when I think about the amount of memory being put on to the current crop of video cards, I doubt whether it'll ever get used.
For this reason I think that an AGP aperture size of 64MB is fine since:
1. It'll probably never be used;
2. System performance is not increased by having a large AGP Aperture.
Cheers.