1333

Is it safe to use two different speeds RAM sticks ? currently I am using 2 sticks of corsair 4gb (1600), I want to add my unused 2x2gb corsair 1333 on the other 2 slot. Surely 1600MHz down to 1333. question is should I use it?

I have a Corsair ValueSelect 4GB 1333 mhz RAM currently installed on my system (H61 Motherboard).. Yesterday, a friend gave away his EVM 2 GB DDR3 RAM to me, but Im skeptical on installing this on my system as improper configurations can cause problems.. Can anyone provide any links how to...