The operative word in your question was "system". In a system, everything has to work together. So it seems that the most important component would be the ability match or put together components that are compatible. In a true "system" all the components will be important. When trying to find out which is the most important, you will have to decide what part of the system you are willing enhance and what part of it you are willing to sacrifice performance if you are willing to spend less or buy lower quality in a particular component. ThAt being said, the decision will be based on what is more important to you.
