The requirement for AGP memory space shrinks as the graphics card's local memory increases in size. This is because the graphics card will have more local memory to dedicate to texture storage. So, if you upgrade to a graphics card with more memory, you shouldn't be "deceived" into thinking that you will need even more AGP memory! On the contrary, a smaller AGP memory space will be required.
It is recommended that you keep the AGP aperture around 64MB to 128MB in size, even if your graphics card has a lot of onboard memory. This allows flexibility in the event that you actually need extra memory for texture storage. It will also keep the GART (Graphics Address Relocation Table) within a reasonable size.