The best way to go are components up front and a sub in the boot.
Make sure you have some good quality speaker cabling, because poor quality cables can let a system down.
You can get 6x4 speakers for the shelf supports, but they arent really worth it.
I have alpine SXS-1357 Type S speakers and they are brilliant, the mid/bass drivers are a direct fit in the door grills and the tweeters fit easily where the dash speakers should be.