I run DDR2-533 and it was running in a 3:4 ratio at 1.95V unstable. My processor was overclocked to 3.402 GHz on a 247 base clock. I was get sick and tired of damn thing resetting every second. So, I went into the BIOS and I dropped the voltage to 1.90V and I changed the ratio to 1:1 so now my ram is running at the same speed as my processor. Voilla, I'm stable.
What bothers me and makes me laugh is that when I changed the ratio from 3:4 to 1:1 my memory speed dropped yet my bandwith went up by some 600 MB according Sandra. Secondly, dropping the voltage should've triggered instability yet my system is stable, wow!