New to Gaming PCs

I am about to buy an alienware m17xR4 and I'm trying to find out if its worth it. CPU is i7-3740QM, CPU speed is 2.7GHZ. Isn't 2.7GHZ slow? when I look up the specs online it says this model has a 2.7 GHZ but runs at 3.6 GHZ with turbo. Is the turbo built in? is this PC good for playing first person shooters? is it worth $300 (what my buddy is selling it for)

You did not say what GPU it has GTX 680m was pretty good for 300 I would pick it up in a heart beat worth every penny. Should play most titles on High with no problem.. good luck on your choice.