|Most boards allow you to tweak the voltage settings. How much depends on the board.|
My opinion of that practice is that the manufacturer is juicing the chips to gain better specs.
IMO you should buy RAM that actually meets JEDEC voltage standard of 1.8v for DDR2. That way, you can juice it yourself to overclock the RAM, if you have the desire to.
Look at the link below. All the RAM listed in that link runs at 1.8v. You can filter further to select the speed, capacity, etc.