When you turn on your PC,press "del" and it will go to BIOS,in there go to "Memory" tab and you will find the setting there.
BTW did flashing your BIOS help you ?
No, it didn't help, remained the same problem. As I read on many forums, this is the most common problem on mentioned motherboard. For that old motherboard it is not worth buying one module of 2GB/800MHz which costs around 40€ = 60$, because new generation motherboard costs around 50-60€... :-(