Oooh, I'd be a bit more careful before changing any speed in the BIOS before you know what the hardware supports. In this case, you're still not sure what kind of RAM you have. It is possible to rev up the FSB (frontside bus) without having the memory that supports the faster speed. Also on older motherboards, there isn't a way to separate the FSB and the Memory Bus speed. So when you increase one, you increase both. That can be dangerous not only to your memory, but your PCI components as well (connected through the Southbridge).
I'd immediately change the setting back to 100 for now until you can find out more about what you have. I've never had this kind of problem before, but I'm sure there's a utility out there that can help...
~cdogg
"The secret to creativity is knowing how to hide your sources."
- A. Einstein