jaco - pavlos, please see here:Abit AB9 Pro FAQ
I had the OCZ PC2 6400 Dual Channel set and it also would not boot on my AB9 Pro with the factory v1.3 BIOS. I bought Patriot PC2 6400 Low Latency memory and it booted successfully. After updating the BIOS to v1.4, I set the memory to 4-4-4-auto, raised the DDR2 voltage to 2.10, set the memory to operate at (1:1 ratio) and now operate my E6400 at 367x8 (2.94 GHz).
I have even reinstalled the OCZ PC2 6400 and it is now seeing both types of memory. However, I have not booted the machine using only the OCZ PC2 6400 memory so I can't say if this would work.