Hi, I know that this thread is old, but I actually have the same motherboard as you Asus P5G-MX and i ordered the same intel E5700 core 2. It has not arrived yet tho, and seeing that you had a problem makes me worry. I see the E5700 is not listed as compatible with P5G-MX, only to P5G-MX/1333. So could you please tell me if it worked for you with the bios update? or did you changed the hardware?
I would really appreciate if you reply as soon as you can. Cheers.