There is a law, Moore's law, that states the number of transistors that can be placed inexpensively on an integrated circuit doubles approximately every two years. So far that's been pretty much bob on. In fact the entire industry is constantly upgrading, which can present somewhat of a dilemma.
The mantra for anyone who has bought a technological product has to be 'don't look back'. We're only semi-joking when we say that your PC is out of date as soon as you get it out of the shop. So it's normally wise to buy what you can afford. The second your purchase is made, the clock is ticking. Processor and graphic card speeds will be increasing, faster ports will be invented, new formats will be created. The more cutting edge your willing to go, the longer you will be able to wait before having to upgrade.
Of course, careful maintenance of your machine will lengthen its lifespan. A PC with a full hard drive will run considerably slower than one with space. Even just keeping the hardware clean can increase its speed, as components covered in dust run hotter and therefore slower. As a well maintained car will last longer than an uncared for model, so with computers. More on maintenance another day.
In the meantime we all have to find our peace with the fact that the machine you're reading this on is out of date. You are no longer new and improved. You are old and inferior.