I'd say it's just your Mobo finding the nearest divider to your native memory speed, and because you're only running a 250FSB, it's multiplying it by 3 to get 750, which when divided by 2 gives you 375, which is what CPU-Z is reporting you're memory is running at. Make sense?
If you go to 300FSB as planned, then your memory divider will probably go to 2.5, giving you 750, and a memory speed of 375 again.
I'm not familiar with your motherboard, but you can usually set memory speed manually, and run it unlinked from the FSB in many cases. That way, you can manually set your memory speed to 400mhz, and also tweak the timings a bit tighter so it's running at its specification.
You're going the right way though, try and get it stable at 300FSB, and then you can play around with the memory speeds/timings to eak a bit more performance out of it.