For a gaming machine, be it a laptop or desktop you need to look at the following components.
If you are a gamer and play high end games, then you will need and INTEL processor like a core i7. If finances are limiting go for an Intel core i5, but it may not be as fast as a core i7. Do not, go for a machine with an AMD processor as they are not fast enough.
2. Graphic Card/ GPU (Graphics Processing Unit)
A good graphic card is a must, and you have a number of options and popular selections included AMD Radeon HD and NVIDIA Geforce GTX gpus. Any of the NVIDIA GTX 700 and 800 series is also good.
Get as much RAM as your finances will allow, preferably 8GB. The minimum threshold is 4GB for a good experience.
4. Hard Drive
A large Hard drive is a must, since games require a lot of space, at least 1 TB. If you end up having to buy external hard drives the gaming wont be as fast as it should be.
I my personal opinion it is better to buy a gaming laptop than to try and pimp it yourself, unless you are really skilled or you have a skilled technician to help you.I repeat that is my own preference