In the past I used to build my own computers from components to get the performance and spec I wanted.
recently I've lost touch plus computers have become a commodity item in the sense that almost any computer will do all but the most specialised task pretty well.
with this in mind if you are considering buying a computer it now makes sense to give serious consideration to a refurbed machine. In general big companies lease their computers and so after two or three years the machines go out the door at end of lease. These machines are than often sold on as refurbished and they can sometimes have decent spec and good prices.
as a recent example I got an HP Z220 workstation (i7, 8gig ram, USB 3 etc.) for £320. This is a pretty nice workstation that was probably £2k two years ago. For really heavy lifting even Z820 workstations are available, as well as run of the mill desktops, and prices for a useable machine can range from under £100 to a fully specced Z820 at probably £3k.
Not it for everyone I know but I've found that a lot of people don't know just the range of refurb stuff that is out there and that big savings can be made from cheap desktops for Internet browsing right up to full blown workstations for high power graphical processing and modelling. Don't buy a computer until you've at least taken a look and given it consideration as an option.