AGP

AGP memory simply measures how much memory your video card has. My GeForce3 has 64Megs, for example.

Systems with integrated video often share AGP (video) memory with system memory. You can check your BIOS to find out how much memory is shared.

AGP has nothing to do with the video card memory. AGP memory is a part of the normal system memory that is simply treated differently by the chipsets memory controllers. It is taken out of the normal cached virtual address space, and given the possibility of direct DMA over the AGP port, by means of special circuitry in the chipset. As it is not considered as standard RAM anymore, you have to allocate it separately. Once you have a pointer to it, you can access this memory as usual, with the difference that it is uncached. This means, that read operations will be extremely slow, while write operations are fast. Basically, you should treat AGP memory as write-only. The graphics card can directly access the data in the AGP memory over the AGP bus, without help from the CPU. Note that while AGP memory is treated differently, it is still a part of your normal system RAM - it is totally unrelated to the onboard video memory of your card (which is usually called VRAM).

Also your AGP slot is connected directly to your NorthBridge while your PCI/ISA slots are connected to your SouthBridge. The inclusion of the AGP slot into the NorthBridge yields faster access time for your graphics card because your RAM(if your onboard video RAM is exhausted) is also connected to your NorthBridge as of well as your processor. Your PCI slots will have to cross your SouthBridge and the NorthBridge to be able to communicate with your system RAM.

I think the ideal transfer rates for your AGP bus at 4x is around 1.06GB/s of course that is an arbitary number but something like that. With that much theoretical data to transfer around, it kind of makes since why a graphics card would need its own dedicated bus port.