Im really new about RAM knowledge and find the best performance laptop ram and best price, but I wanna ask,

I have a different brand RAM but have the same memory (4gb) really not remembering on Mhz, but is it okay if I installed this RAM with other if they had different (Mhz) and brand? I have new laptop Asus a45V with win8 and have corei7 with nvidia geforce gt 645m. I wanna buy that RAM (hopefully at least 1600Mhz) because my laptop have 1 empty socket RAM ready to welcome a new addition.