Computer manufacturers such as Dell, HP, IBM, etc, have come a long way in providing consumers with a number of options in building a custom computer system. These computers can be quite good for the typical user but if you really want a system that you can use for years after you buy it then you really need to consider a custom-built PC.
Most manufacturers use proprietary hardware with their PCs and this can make it extremely difficult to upgrade your PC as the various components become obsolete or are not compatible. Examples include the CPUs, RAM, network adapter, sound and video adapters. While some of these components can be upgraded after the initial purchase, you will often find that you are very limited with what you are able to upgrade.
Building your own PC allows you the most flexibility and when you look at the quality of the components used in the system, the cost is often less than buying a pre-built system of the same specifications. Another advantage of a custom-built PC is that you can start with a very basic system with a low end CPU combined with maybe a mid-range system board and minimal memory. You may also want to just start out with a generic sound card and video card or you could purchase a system board that has on board sound and video (not recommended unless you are totally broke or short on cash at the time of purchase). You can always upgrade to a better sound & video card later. You will want to do this if you decide to play some graphic intensive games on your PC.
Now you have a basic custom-built PC. When you have some more money, you can then install more RAM which is one of the best ways to enhance PC performance. Eventually, you may decide to upgrade your CPU and you may be able to do this once or twice depending on how much you are willing to spend. Best practice is to just upgrade the CPU to the best one that the system board will support. A custom PC like this would probably last you 1-2 yrs before it "feels" like its outdated. So, instead of going out and buying a whole new PC, you would only need to at most swap out the system board and processor. You can still use the same sound card, video card, computer case, RAM, CD/DVD drive, hard drive and of course keyboard/mouse. The only thing you will have to do though is probably reinstall your operating system and applications. The reason for this is that the current setup may or may not support the changing out of a system board which can cause the OS (either Windows XP or Windows Vista) to crash. Best practice, is to back up all important data to a CD/DVD, and then swap out the system board and reinstall your OS. Once that is back up and running, you can then restore your data.
Do you have a custom-built PC? What are your thoughts about them?