Team-BHP - Need help with audio for NHC

Dear All
i read a review in another website and thought i will take the help of pros here too to get some of my doubts clarified.

My setup is this.
New Honda City
Head unit Sony CDX-GT460US
Front speakers.Sony HT170 NT-component,peak 240W ,rated 60W.
Rear Sony xs6948- PEak -420W ,Rated-100W
The music is good enuf and am enjoyin the traffic at bangalore already
My Car accessories dealer was pushing me to go in for a 4 way AMP and sub but i was kind of waitg for it to be a little easier on the pocket.He gives me great prices compared to the general market tho with miniscule labor charges.

My questions are
1. the power wattages in my car add up to abt 650-700W peak power.My friends say that it will be an imbalance with the headset only abt 52X4 and cud end up dmaging the speakers or head unit. .is there any room for worry?
2. I was thinking of putting in a amplifier and a sub later wen i cud afford . i already have a decent effect in the car..thinkin if i really need to invest.

3. if yes do u think i shud go for a 4 way AMP and what rating on the sub. i prefer Sony bcos i hv always been a sony man given a choice.

4. also is there any issue in really blastg music on the car.? i am worried if any issues will form with the speakers without attaching the amp.

thanks
rgds