I've been looking the Gigabyte GA-MA790FX-DS5 BIOS F6. bios's checksum. Both files the original and the modded one have the same 8bit chechksum=00. What is cousing the checksum problem, I cant say. Maybe both chips needs to be flashed at the same time.
Prior to flashing, the PC checked the checksum of the new BIOS file and in each time it came up with a different figure. If I remember correctly, the only time it said checksum=00 was with the original F6 BIOS.