How does AGP utilise system memory?

[Bojo]

Does an AGP card have access to all available free system memory?

Is the card bus faster than the system bus?

If I have a 16 MEG card with 256 MEG system memory is it much better than say a 32 MEG card with only say 64 MEG system memory?

 

[AndyHui]

AGP only has access to the amount specified in the BIOS as the AGP Aperture size.

If you have a 32MB card with 256MB of system RAM and a 32MB aperture, it's not really right to say you have a 64MB card with 224MB of system RAM. The system RAM is dynamically allocated....textures are only loaded when needed, and there are not that many cards that regularly pull textures direct from memory because of the less than spectacular performance.

Saying that you have a 64MB card with 224MB of RAM indicates that this is permanently the case, which is not how it works. Let's just say that normally the AGP aperture is "reserve area" for textures that spill over from the video card's onboard RAM.