The 10 is the Multiplier so you take 240 x 10 = 2400 which means your CPU is now running at 2.4Ghz. And yes when you OC your CPU it does require more voltage. My motherbored automaticaly changes the voltage depending on how much i OC the CPU.
You can also take a different Multiplier and times it uselly by 4 to get the FSB (front side bus) 240 x 4 = 960Mhz
(i dont know if on the AMDs you have a different number Multiplier for the FSB