There's quiet a few options you can go with, it depends on your budget and taste at the moment, as well as what you want in general. Stock location of the front speakers SUCK! I would recommend mounting speakers in the doors, and for a better front stage, go with a 6.
For the rear towers, you can put up to an 8" back there, a 6 will fit as well.
email me if you have more questions, and check out my site for pics.