An article about it
http://www.tweak3d.net/articles/aperture-size/
their definition------> Generally measured in megabytes (MB), AAS is the amount of system memory (RAM) shared with an AGP graphics card in order for it to have more memory to process textures and other visual data.
So I guess that means if you have plenty of system RAM that a bigger aperture size is ok....though I havent read the article. Nowadays we all have lots of RAM so 128 aperture may be good for the more detailed games etc.
Maybe some true gamers will respond and let us know the real deal.
JP