that's a complicated question
The mega is fed 12v from the ramps via D1, the mega really doesn't like more than 12v (the voltage regulator cant handle it and will cook itself)
For the ramps board itself it depends on what capacitors are installed, the caps are meant to be 2 * max voltage, but most ramps are only fitted with 16v caps so even 12v is pushing it. Some ramps come with 35v caps.
12v is fed to steppers, mega, fans, hotends and heated bed
Some fans also don't like much in the way of over voltage, steppers are fine, the mega ive already mentioned, hotends and heated beds are fine.
Personally I use a Taurino Power from reprapdiscount, these have a better power supply and can take up to 35v
You still have to think about all the other 12v components.
