Too often people build a new PC because the believe the hype that if your system is not newer than 2 years old you can not play the most current games. This is not always true, many older computer builds can still give a satisfactory experience. Until recently my computer was playing the games i wanted to play at medium settings and 30fps (which i was happy with) and i was using a core2 duo Cpu , 4gb ddr2 ram, and gt 220 Gpu (6+year old rig). when my Cpu finally gave out due to the socket being obsolete i found it more cost effective to build a new pc.
When should you upgrade your PC?
1. When the games you love to play or applications you need to run fail to run properly.
2. When a part breaks or is no longer compatible with your needs.
3. When you have extra money and want to get increased performance from your current machine.
When should you build a new PC?
1. When the core parts are so outdated that finding compatible parts will be more expensive than more powerful current models. (cpu, gpu, motherboard, ram.)
2. When a part breaks and the cost of diagnosis and repair is equal to or greater than the cost of replacing all the core components.
**Remember when thinking about upgrading vs. building new that the only true reason to do either is when the experience your PC provides is no longer enjoyable.**
For help with builds/upgrades look into my other 2 tutorials "building a gaming pc for your budget and level of gaming" and "qualities to look for when buying pc components"