If you are that lazy to build your own PC and save money, then go ahead, buy a pre-built one.
i7 4790k is a quad core with hyper threading. It will be noticeably better in applications or games that do not use more than 4 cores, but a bit slower while video rendering and video editing.
i7 5820k is a 6-core with hyperthreading. It will be worse for things that use 4 or less cores, but will be better for video editing/rendering.
It all depends on what you want. Also the RAM is real cheap so if you want to go with 5820k because you do tons of multitasking like editing, streaming, you can always buy more ram in a store for $100 (16gb RAM)
If you are just gaming, then the first one is your choice