Aperture size is a very deceptive figure. These days, you shouldn't set it to more than 1/4 of your RAM as a rule of thumb, but it should be similar to the size of your video RAM (eg 128MB or 256MB for a video card with either 128MB or 256MB onboard).
AGP aperture size is the amound of RAM that you are allocating as dedicated to transferring to/from the AGP bus. The problem with AGP aperture memory is that generally speaking, you want to use the AGP bus to transfer memory to system RAM as little as possible, since system memory pales in bandwidth to that onboard AGP/PCI-e video cards (6.4 GB/s for dual channel PC3200 vs. 25GB/s and up for high end video cards).
I used to go bananas on my AGP aperture size as well before I figured out what it meant.
Now I leave it planted at 256MB of my 1GB of system RAM. Anything more is a serious waste and saps useful memory that you could be using to run more programs 😉 .
kmmatney 64MB is a bit too low for any modern card. I would recommend 128MB to be the minimum, especially if your video card has 128MB of RAM or above.