ehh, that's entirely up to the manufacturer of the RAM.....if you want info on memory perhaps it's a good idea to include what memory you are wanting info on.
If it's rated at DDR500 speeds then the manufacturer will generally have specs that'll tell you at what voltage it needs to run at those speeds.
The dimm slots don't regulate the voltage, the BIOS does...2.6-2.9 is common options but, you'd have to check your BIOS for that information