The main problem is that i can't set the FSB:MEM divider the 1:1 in my bios. The option to do so is just not there.
I haven't had one of these mobos before, but I have had a lot of other Asus mobos. I have so far never seen one that has a BIOS option 'memory divider'.
Instead, what these mobos have is an option to set the memory clock speed (or memory frequency).
The chances are that you currently have it set to AUTO. What then happens when you OC is that the motherboard will automatically drop the memory clock frequency to 166 (from 200). This translates into a 5:4 divider (more or less).
If you want to scale up the memory clock with the FSB overclock 1:1, then manually set the memory clock to 200MHz.
I'll try and check the manual for your motherboard to see if I can provide more detail on this.