Laptop RAM upgrade?

I’m trying to upgrade the RAM of a laptop. It now contains 2 x 2GB DDR3 1033MHz 204-pin SODIMM modules. I want to upgrade to 2 x 4GB DDR3 1600MHz 204-pin SODIMM modules. The memory voltage is now 1.5V. Does it matter if I use 1.65V modules? Or do I have to use modules with the same or lower voltage? What happens if I use modules with a higher voltage?