The standard for DDR2 is 1.8v and all DDR2 will run at that voltage. Some higher performance memory is rated for lower latency's and or faster speeds at higher voltages. These higher voltages depend upon the perticular module as they can be rated anywhere from 1.9v-2.4v. There is a chip on the memory module called a SPD chip where the factory settings are saved, some modules have EPP or Enhanced Performance Profiles where more than one setup is saved on the chip. All DDR2 memory will work in a DDR2 compatible board you just have to follow the speed ratings set by the manufacturer, in other words don't be suprised if you have trouble running DDR2 1066 in a board rated for DDR2 800.
Lower end motherboards might not be able to adjust the voltage or have the settings in the BIOS for you to do it manually, the RAM will still work but not at it's highest speed. My memory is from Mushkin and is rated at 1.8v 667 3-3-3-10 1T and 2.1v 800 4-4-4-3-10 2T just to give you an example.