As said in the previous posts the sound you get from the speakers in the front is far more important than the sound coming from speakers in the back, having said that I do believe that rear speakers are worth having and provide "rear fill" and compliment any well set up system. The one thing I would advise is that the rear speakers are fitted into a MDF shelf as this gives them a solid platfrom and vastly improves the way speakers sound, if your not running a sub then 6x9's or 7x10's are the boys to go for. If you are running a sub I would just upgrade the standard 6x4's so you have a bit of mid range and treble in the back, to be fair its all down to personal choice really and I would try to listen to other peoples systems to give you some idea of what to expect from various set ups.