If it is purely for gaming and only running 1 game at a time, then the i5 series is the place you want to start looking, as they provide a good strong processor clock speed and are VERY overclockable. If you want to be editing 3D objects and/or want to be editing 1080p video AND be running multiple games, then the i7 series is where you want to start looking. The i7's enhanced processing power, hyper-threading technology, turbo-boost technology, and 4-cores/8-threads allows for massive tasks to be run and lots of em to be run at the same time. I personally use the i7-3770k on my gaming rig, simply because it fit my budget, and it is very powerful and flexible, although I have never tried overclocking because i think that anything over 4gHz is a complete waste of time and CPU life. AMD processors tend to not be as powerful for the same price as Intel processors in my experience, and although intel CPU's are not very upgrade-able, they have a wider variety of CPU's to chose from and they are very specialized until you reach the 3rd gen i7 series, and AMD uses a different CPU architecture than Intel does, which means that no matter the GPU, some games will run better than others.
Some advice: Overclocking does improve performance of a CPU depending on how much voltage and frequency you set, but I HIGHLY RECOMMEND AGAINST overclocking on a laptop, even a gaming one. This will increase temperatures and will reduce CPU life faster than on a desktop, and in some cases, can melt the cpu, motherboard, or even the chassis. NEVER overclock if your BIOS is locked, as it is likely locked for a reason, but in most cases, especially if you custom-built your computer, the BIOS should let you do just about anything.