I have no idea in ram/random access memory is it okay to upgrade 667 mhz to 1333 mhz?

I have 2x2gb ram ddr3 667 mhz on my pc, can i upgrade my ram up to 2x4gb ram ddr3 1333 mhz? I am planning to buy new pair, and i am newbie in pc building/pc parts. I hope someone will help me.