but its wrote in stone by motherboard manufactureres everywere
Also not true. No where in my manual do they state such a formula for determining aperture size.
If you upgrade to a graphics card with more memory, you shouldn't be deceived into thinking that it will require even more AGP memory. Aperture size is inversely proportional to card memory. Actually a smaller AGP memory space will be required.
Note that the size of the aperture does not correspond to performance so increasing it will not improve performance.
Still, it is recommended by most to set the AGP aperture to around 64MB to 128MB in size.
Most cards require an AGP aperture of at least 16MB in size to work properly. This is probably because the virtual addressing space is already 12MB in size. In addition, some software has AGP aperture size requirements that are not really specified. Some future games may use larger textures, but for now none use more than 32MB, which translates to an aperture size of 64MB.
Please note that reducing the AGP aperture size won't save you any RAM. Again, what setting the AGP aperture size does is limit the amount of system memory the AGP bus can appropriate when it needs to. It is not used unless necessary. So, setting a 64MB AGP aperture doesn't mean that 64MB of your system memory will be appropriated. It will only limit the maximum amount of system memory that can be used by the AGP bus to 64MB.
Now, while increasing the AGP aperture size beyond 128MB won't take up system RAM, it would still be best to keep the aperture size in the 64MB-128MB range so that the GART (Graphics Address Relocation Table) won't become too big. As the amount of memory on graphics cards increases and texture compression becomes commonplace, there's less of a need for the AGP aperture size to grow beyond 64MB. Therefore, it is recommended that you set the AGP Aperture Size to 64MB or at most, 128MB.
With my 128MB GF4 TI4200, using 3DMark 2001 as a reference, going any higher than a 64MB aperture size makes no difference. And although not conclusive, it actually seemed a bit off the mark when set to 256MB.
But as with all computer related things, one size does not fit all, your mileage may vary.
The above information has been culled from various sites and discussions.