If it's for standard office use or similar, just buy a pre-built one. You can get nifty tiny, silent cases that are vastly overpowered for anything you might want to do with it. If you need more power, I would select the components myself, but leave the grunt work of building to a retailer. Where I live that costs about 75 euro and gets you three years of warranty, so it's a great deal.
Pre-built gaming systems tend to be unbalanced, throwing lots of money at high-end components where only very marginal gains can be expected in actual real-life performance. You don't need "black" CPUs or hand-picked memory, and you don't need dual graphics cards either - unless you enjoy paying through the nose for a problematic component that will be outgunned six months down the road anyway.
As for the notion that you need to build one yourself to prove your manhood: look buddy, unless you soldered your own graphics card or whatever, all you are doing is clicking together some premade components. A monkey could do it.