It will be the bios.
Also max ram you can have is 2GB am sure you know, this though is a limitation of the chipset. Because the bios remaps the RAM even before the OS boots, when booting the OS will only be able to access the V-ram through the onboard graphics...if your MB limits the max you can remap as 256Mb then that is it. No point trying to find a patch or hack or work arround...if you need more graphics RAM to play a game,etc. buy a PCIE graphics card if you havent already filled up that slot.