The i810 chipset has an optional i810DC.....Display cache, 4MB fixed on the chipset as a frame buffer for the onboard i740 video. This is not the same as an AIMM....AGP Inline Memory Module. The AIMM functions much like a COAST module; it is simply a PCB a memory chip on it, which is plugged into the AGP slot to provide frame and z-buffer.
In the case of the i810 chipset without the DC option, and the i815E chipset, video data is stored in the main system memory and must share bandwidth requests with system data. Having an AIMM is a very cheap way to increase performance; not only by giving the onboard video a dedicated frame buffer, but also meaning that video data no longer needs to share bandwidth with system data. Performance has been seen to increase nearly 20% simply by adding an AIMM.
<<graphics coprocessor >>?? You mean an onboard Transform and Lighting Engine? This is a relatively recent development in consumer level cards beginning with the nVidia GeForce family. Not all cards have an onboard T&L engine.....currently the ATi Radeon and nVidia GeForce lines are the only ones to implement one which is actually of any use. The Matrox 550 implements an extremely limited one for specific uses, while the KYRO does not have a T&L unit at all.