Check this out from Lost Circuits---
AGP Aperture size
The amount of main memory that is allocated for direct memory access (DMA) of the advanced graphics port or DiME (direct memory execution). The hypothetical advantage of the AGP design is that the graphics engine can use the main memory as part of a unified memory architecture (UMA). In this case, the graphics adapter can access textures directly from the main memory (DMA) and, more importantly, process the data directly from and to the main memory, bypassing the on-board graphics memory. This feature requires a certain amount of system memory to be allocated for graphics operations. Too little will result in poor graphics performance, too much will fill up the system memory and take up space required for other operations.
With the increase of on-board graphics memory (currently up to 128 MB on the video card itself) this option has lost its relevance. Were the original recommendations to set the AGP aperture to about ½ of the total system memory, a good approximation now would be:
Total system memory / (video memory [MB] /2)
In other words, if you are running 128 MB system memory and have a 4 MB video card, the optimal AGP aperture would be:
128 MB / (4/2) = 64 MB
For a 8 MB video card, the equation would be:
128 MB / (8/2) = 32 MB
and for a 64 MB video card:
128 MB (64/2) = 4 MB
Please note that this is only an approximation to explain the principle. In real life situation, the AGP aperture should be kept at a minimum of 16 MB for most applications, a few others, particularly benchmarks like 3DMark2000 require at least 32 MB AGP aperture
So if my system memory is 512, divided by the video memory of 128, divided by 2 I should use a aperature value of 8 MB????