You can use any speed RAM in the laptop, as long as it's the right type of RAM. If the laptop can't run it at that speed, it would simply downclock it, or loosen it's internally set timings, to allow the RAM to run at that speed. I did a similar size/speed upgrade on my Alienware M18x and it had no issues with it.
If you want to be SAFE, I could get chips the same as what you have now, just bigger size ones, i.e. if you have 2GB, get a 4GB set of the same speed, etc. That's the safer way to do things.
CPU: i7 3930k @ 4.6GHz 1.29V (Corsair H110-cooled) ~ Motherboard: Asus RoG Rampage IV Extreme
RAM: G.Skill Sniper Gaming Series (4x4GB 2133MHz, CL9) ~ PSU: Corsair AX 1200W ~ Case: Enermax Fulmo GT
GPUs: 3x HIS R9 290X Reference (Core) Editions in TriFire, Max Load Temps: 71C/70C/71C during OC testing :-P