Set up the voltage for your memory, and possible the north bridge
just do a little at a time then run mem test to ensure stability for every increase.
You mention you are changing the FSB - that changes CPU speed. I suggest you lock down the CPU to a known stable speed, and just change the memory bus speed (bus between the Northbridge and memory). Then test with memtest86 as you try different memory speeds.
What timings are you using?
Should I be using certain timings? Also, when I change my DIMM voltage in my BIOS, it doesn't show up as changed, in CPU-Z for example. It still says 2.5v. Is it locked?
I'm using the auto/default timings.
Timings for Buffalo PC3200 are 3-3-3-8. This is normally set by the BIOS when using Default as the setting in the BIOS for memory.
Use CPC off, or 2T timing, when using two or more modules at a time.