In some cases, all the PC manufacturer does is put them all together, glue their badge on the case and throw in a nice instruction manual. This modular nature pushes the price up. However, modularity gives you, the buyer, some freedom. You can specify exactly what goes into your machine, which gives you control over the price and also the performance. Here's what to watch out for.
Speed is all important, something which is measured in megahertz.
However, by the time you read this it might be twice that. Or three times. It's a fast moving market. Then there's the manufacturer of the processor. If you believe the adverts then Intel can't be beaten.