Hi Micklemass. You set the AGP Aperture size in BIOS. For your motherboard it is probably under the Advanced Chipset Features menu. Change the size one step at a time. You probably have the option of 4, 8, 16, 32, 64, 128M. If your motherboard is fairly recent you probably have sufficient graphics memory to get away with fairly low AGP aperture, so see how far you can go before performance deteriorates.
Also consider adding another stick of memory--these days it is not too expensive and if you have a need to run up to date applications it might be wise to upgrade a little,removing your "problem" at the same time!