It's been a while since I played much with S939, but IIRC, what you are experiencing is pretty common.
Most S939 boards don't play nice at all with 4 sticks of memory, and depending on the core of the 3400+ chip (I'm guessing it's venice) I'm pretty sure that it's built right into the on-board (it's in the cpu) memory controller to drop mem speeds down for 4 mem sticks.
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