Building one is usually a lot cheaper than buying a pre-made one, and they usually come out better in the end, but if you're like me and have no skill whatsoever then just go with a pre-made one. It'd save you a lot of time and frustration.
it is cheaper to build one yourself due to the fact that all the "generic" ones are that, generic and accesible for everyone with prices ranging according to the demand of the product, which is a lot and more expensive because lets face it, all they care is about money and not quality, thus are not comparable with something you build for yourself according to your own demands knowing which part to buy with their own range of price
of course you could always buy an alienware and get on with it
And if your going to buy a monitor. Try to get a Dell or a Samsung. I have had good experiences with both monitors. Even though Dell overprices thier junk computer they still make great monitors.
You can find everythin you need on newegg.com. I did.