Agp apeture size alows the video card to use system ram as soon as its own ram runs out. In your case hardly ever. Most people set it to half the physical ram they have or double the amount of video ram they have. I have tested it at 128mb and 64mb on my machien and it makes no difference. I found a really good artical on it a while back I'll see if i can dig it up for ya.
Edit: Its kinda old, but it gives the info you need.
Make sure u have it set to at least 64mb, I have found anything lower than this results in glitches.