exios is right, as far as how quickly a computer can be built.
It's always a good idea too read as many reviews as possible once you decide what you want to go with.
first you decide what cpu you'd like to get, amd or intel,
then you decide what motherboard you'd like to use.
once you know which motherboard to use, you'd look for reviews on
www.google.com on that board. using the words motherboard review motherboards-name-here. Usuaully in these reviews, they'll have whats called a testbed, which they use multiple computer parts to test out that motherboard. It can give you an idea of which parts you should use to get maximum performance from that particular system.
Never rush into buying something, always read as much info on the product before you decide to build!
happy building, and good luck.
