A7N8X 2.0 Memory Problem...

My old CPU was Athlon XP 2400+ 266Mhz. The motherboard is Asus A7N8X2.0 Rev
1.xx with no BIOS update. (I am not sure if the rev is same as PCB version.
The motherboard says nothing about PCB version...) I have 2 512MB DDR 3200
memory modules.

The problem came when I upgraded my CPU into Athlon XP 3000+ 400Mhz FSB.
The computer won't boot up with long beeps, and when I switched back to the
old CPU, the problem still remained.

I removed one memory module and put the other one in socket 3. (The
sockets are black colors only!!!!) It finally worked. But...hey, I don't
want to waste the other memory module.

So....what's the problem? I have no idea if the PCB is either 1.xx or 2.0.