AGP Apeture tells the PC how much physical memory can be used for video, in addition to what you have for your card.
Three possible scenarios...
1....
You set the apeture too low, and your graphics do not meet maximum performance.
2....
You set the apeture too high, and the program does not have enough memory to run properly. Thus, not giving you maximum performance.
3....
You set the aperture just right, and everything works perfectly. Benchmark scores increase and frame rates are better.
To perform benchmark tests go to...
http://www.futuremark.com