Save your money and concentrate on improving your front speakers. You'll probably notice no substantial gains from replacing the rear speakers (unless you're adding a sub).
As you're adding a sub to cover the bottom end, 6.5" components would be the best choice for SQ.
Put that money you'd be spending on rear speakers towards a moderate amp for the front components. The difference from running speakers with an outboard amp as opposed to the weak HU amp is exponential.
