I have been building custom computers for over a decade. While agreeably, this has been mostly for friends and family, alot of research goes into the components that make make a great gaming machine or high performance workstation.
Once of the biggest complaints most of my customers have is they want a computer that they wont have to upgrade for "x" number of years. Those computers are not generally sold in retail stores. Why? Too expensive? More expensive certainly. But much higher quality components.
Case in point.
Most desktop computers have integrated motherboards. That means the video processing shares the main processor for tasks and takes away from the main system memory which leads to a loss of performance. Combine this with low grade memory and your "performance machine" is anything but.
I do alot of research before putting any components on a quote for your system. Optional components are spec'd in based on what your expectati and budget is.
Let me build a computer for your needs.