The PC you need may be, and probably is, very different from one appropriate for your friend, neighbor, or relative. Maybe you only want to write a few letters or e-mail messages to your friends, surf the Internet once in a while, and balance your checkbook. Each of us has different needs so we also need different PCs. On the other hand, don't scrimp so much that in six months you will find yourself with a PC that won't do things you actually need to do. Your needs may vary.
Many decisions have rule of thumb guidelines and deciding whether to buy or upgrade is no different.
Four steps to computing nirvana. Of course it really isn't quite that simple. The truly difficult part lies in determining your needs and what level of each PC component is needed to meet those needs. It can be difficult to determine your needs if you aren't fully aware of what PCs can do, and the software currently available to run on them. It is also difficult to know what internal PC components to specify if you are unfamiliar with the terminology and functionality, to say nothing of the difference in brand names.