Well, for starters AGP means : Advanced Graphics Port, and is based upon the PCI rev 2.1 specifications. There are however quite a few differences wth PCI : The AGP bus runs at 66 MHz while PCI runs at only 33 Mhz (This on systems with a 66MHZ bus speed) AGP supports data transfer on both the risising and the falling edge of the clock ( X2 mode ), resulting in 4X the PCI bandwidth. pipelined memory read/write operations no sharing of bandwidth with other components like in case of PCI DIME, direct memory execution of textures CPU accesses to system RAM can proceed concurrently with the graphics chip's AGP RAM reads. Allowing the CPU to write directly to shared system AGP memory when it needs to provide graphics data, such as commands or animated textures. Generally the CPU can more quickly access main memory than it can graphics local memory via AGP, and certainly faster than via the PCI bus.

Due to the high data transfer rate between the graphics accelerator and main memory, AGP enables graphic accelerators to use main memory instead of local memory for things like typically textures, which can be as big as up to 128 kB. These textures so far had to be loaded into the local graphic accelerator memory to be processed there by the graphic processor. Now these textures can be processed in main memory without a performance impact. Intel calls this DIME, for DIrect Memory "Execute". UMA the 'unified memory architecture' used on low cost boards in the past, where already main memory was used as graphics memory, had two important differences: The main memory provided via AGP and thus called 'AGP memory' doesn't replace the screen buffer of the graphic accelerator as done in UMA. The AGP memory is an addition to it. UMA had to go through the much slower PCI interface. These two differences show why UMA was particularly slow and should make you understand why AGP graphic accelerators should be faster than current PCI solutions.

This leads to the question if you have to use DIME to benefit from AGP. The answer even provided by Intel is 'NO'. You can use AGP without using the DIME feature at all. In this case the graphic accelerator is just benefiting from the much higher transfer rates than PCI. The 'sidebands' can be used, but they don't have to. Without 'sidebanding' the transfer rate is already 266 MB/s, which is double of what a stand alone PCI graphics card would get. Here the access can (as with PCI cards as well) either use PIO or DMA to transfer the data from main memory into the frame buffer of the graphic accelerator. The majority of graphic accelerators will most likely use DIME, thus saving on board texture memory and hence making the card cheaper without loosing performance. Of course these cards should be using the 'sidebands' to enable 'x2' mode. The high end versions will most likely use DIMEL (Direct Memory Execute and Local also). Often used textures would be stored in a (large) on board local memory, less frequently used ones would reside in the AGP RAM. These cards will come with a lot of memory on board, like e.g. the (expensive but fast) Diamond Fire GL 4000 (PCI) with its 32 MB RAM already shows. Even Intel admits that high end solutions will still have a very large local memory, but will be too expensive for mainstream.