Basically it sets the amount of main system memory that your video card is allowed to access if it "runs out" of local/onboard memory. Most times the setting doesn't do much, you could increase it to 256 if you want (some people recommend setting it equal to the amount of your card's memory), but it won't make a difference in 99% of the things you run.
For example, when I used to have a 9700 Pro, the only time the AGP aperture setting helped me was when I was trying to run that Ruby X800 demo - it wouldn't really fit in memory and was little more than a slideshow, so when I increased the aperture to 256MB (I think - or it may have been 512), it ran pretty acceptably then. Also, a friend of mine was having trouble running a demo meant for a 6800GT on his regular 6800 (half the memory), and increasing the aperture did the trick for him. However I'm not really aware of many gains to be had in actual games. 🙂
Discussion Zen 5 Speculation (EPYC Turin and Strix Point/Granite Ridge - Ryzen 9000)
Discussion Intel current and future Lakes & Rapids thread
