How to change AGP Apperture size?

Archived from groups: alt.comp.periphs.mainboard.gigabyte (More info?)

HI!
I've got a computer built around the GA-8I875 Ultra(F9) with an ATI
R9700 PRO,1024 MB DDR 400 and a 80 GB Seagate S-ATA.
Now the question is how to change the Apperture size from 64 to 256 MB
because I can't find that option in the BIOS.On other mobos that option
is in Advanced BIOS or Chipset features but on my 8I875 Ultra that
option isn't there.
Anyone know the solution to this?
Thanks in Advance.