DDR3 RAM. Buy a kit instead of separately single dimms. And make sure it's 1.5V and 1600MHz since there's basically no noticeable difference in getting a higher speed memory. 8GB should also be enough for most users unless you're doing video editing or 3D rendering. Everything in this list will work fine with your motherboard
on the mb vendor web page is a ram qal list. http://www.asus.com/Motherboard/P8Z77V_LX/#support_QVL
cosair/crusial and gskill also have tested ram qal lists.
staying with name brand vendor most time you dont have any issues.
look to pick up the newer low profile ram (1.35v).