As you can see, what you buy today will soon lose its value and overall technological advance. This includes a CD ROM, Sound Card, and Speakers. Most of today's software takes advantage of these components to make computing more enjoyable and user friendly. Software is getting more intuitive for the consumer and requires more and more processing power. A program may say it will run on a certain base machine i.e. a 486 DX33 with 8 megs of ram but let me assure you it will not be enjoyable for most.
Speed is usually a luxury but for some a necessity.
Most all computers you buy will be upgradeable somewhat. An IBM, Compaq, and Packard Bell, etc. will usually have proprietary components making it hard to swap out for a 3 rd party brand. Hard Drives and Ram are the easiest to replace by the home consumer. A computer custom built will use components that are easily upgraded, be sure to ask this before you buy. Most people will not find this issue a concern if they purchase a system with their needs in mind.