How many MBs should this be set to?
About 1/4th of you current Ram. So for example...
1GB = 256mb
512 = 128mb
agp arpeture controls the amount of system ram the video card may use for storing textures. setting this too low on a card with little memory can cause lag, shoppiness, or stuttering. 128 MB is a good choice and is usually the right setting. try 128MB, only mess with it if you experience stuttering.
I was always told by from friend that are really heavy into gaming and stuff that you should set it to half of your Video card memeory. So if you have a 128MB card you should set the apeture to 64MB. Although don't really quote me on that because I don't really know the reason behind why they told me to do it that way.
I have 1.5GB of RAM and a 256MB Geforce 6600 graphics card. If I use the quater of RAM theory should i set it to 375MB??? What is i set it to 256MB?
It should be set based on the amount of memory the video card has, the more memory, the smaller the aperture size.
If that's a vanilla 6600, there's no way you'll be able to run at a resolution with AA and AF that will go anywhere near using 256mb. It doesn't really matter what you set it at because it's only used if it's needed.
Just set it to 64mb or 128mb and forget about it. The only thing not to do is set it below 32mb (it turns of AGP texturing).