"DDR3-1333 is cheap and plentiful. Stick with the regular value RAM and not the enthusiast ones with heatspreaders for best compatibility"..

"its depends on the CPU, for intel cpu, the memory countrol unit is build in the CPU, 2.0G only supports 1333MHz, but both 2.2G and 2.3G support 1600MHz. The detail can be checked from intel website".

1333, 1666 and 2000 are overclockable..

Whatever you've got 'NOW' get more the 'SAME EXACT part numbered RAM.' Do NOT mix-match your RAM! Especially never mix: Speed, CAS timings, and Voltage. The typical difference between 1333 and 1600 MHz RAM is 1~3%
ADVICED to have SAME brand n type if using both slots! otherwise u will be posting same problem as previous users experienced.. 

u can go fot it .. but it isn't worth to upgrade from 1333 to 1666. Performance difference is less than 0.1 %

http://www.crucial.com/upgrade/ASUS-memory...F-upgrades.html

Bro, it's 1600MHz, not 1666.

I'm using Corsair Vengeance 1600MHz in my system now. Check my siggy below. And I never had any problem so far. 

Any of you guys using hybrid hard disk with this lappy?
Might wanna get something like that one day, maybe 1tb + 60gb cache
maybe still not yet exist lol