Pretty certain that it can't be changed once the system has booted. It involves negotation between the chipset and the video card on a level that probably can't happen live.
Odd that you have everything in your signature except for your mainboard. Can't even look up information on it. Anyway, I've never heard of options being locked such as this, unless they were options such as CPU frequency displays or something which you weren't allowed to change. At the very least, the setup screens show you what key combinations if you need to get to another menu.
Incidentally, unless you are running extremely high texture resolutions and high video resolutions while gaming (a 256MB video card can easily handle anything normal that you'd be capable of even playing on that system, 128MB is still good enough usually), the aperture size probably is not going to make any difference. AGP aperture is only needed if the video card onboard memory runs out of texture memory, and is so much slower than video card memory that you might be better off just reducing the resolutions if you are in fact exceeding the video memory.