if you are getting new front speakers then amp them or you are wasting them, simple as.
if you are planning on getting a sub then forget the 6x9's in the shelf, in fact i would forget 6x9's full stop as to be quite honest rear speakers arent needed in a decent in car set up unless you need a little rear fill.
powerfull rear speakers will drag the sound stage behind you and lets be honest, you dont go to a concert and stand with your back to the stage do you!?
by advice to you, if you are on a budget, would be to go to
www.caraudiodirect.co.uk and click "promo packs" - grab yourself the best sub/amp/box/wiring kit combo you can find and then when you have this then start saving for some new front components and a 2channel amp to power them with.
you then have a decent set up!!
if you are looking to put your windows down, make some noise and have people look at you then get some 6x9's and stick them on your shelf, but if you actually value your ICE and sound quality then dont!!
mr scoop2004, if you think that your car audio sounded good then you clearly have never heard a half decent set up... eh will!?