Different parts of a PC need replacing at different paces. Some examples:
- DDR3 has been in the market nearly a decade, if you bought 4GB a while back its still perfectly usable today.
- A 1GB hard drive might be 5+ years old and it will run about the same speed as a brand new 4GB hard drive bought today, and its still plenty big enough.
- A sandy bridge CPU which is several generations old, or even an original i7 920 would still be very good for gaming as performance has only gone up in small increments.
- A GPU from 5 years ago however will be 1/4 - 1/8 the speed of one bought today.
All the components of a PC are gaining speed at different rates. Memory gains very slowly but is about to jump again with DDR4, CPUs are gaining about 10% a year or so, GPUs are gaining about 50-100% every 2 years, hard drives are gaining like 10% in 4 years etc etc. If you just look at the PC as a box then the GPU is going to necessitate a replacement earlier than anything else, so you would replace it in 2-4 years. But you could just replace the GPU in a year or two's time and evaluate what else has changed and upgrade bits as and when that is necessary, saving considerable money overall compared to just buying and replacing the whole box.