Ah... well I dont know how to explain the problem that happens with the 512mb bios.. it displays the top half of the image / whatever is in Vram but overlapping diagonally ... its really odd.. if you want a pic of the problem in action I can accomidate.. also if its not the ram, what is it?
Should be no problem. In fact, there's nothing like a special 256 MB or 512 MB version. The BIOS program figures out which amount of RAM is installed and behaves appropriately.