Car stereo help? Subwoofer?

I am going to be totally replacing my car audio and I need some help deciding what to purchase.

I already have
2 Alpine Type R 1243D 12in 4ohms
2 Alpine Type R 6x9 speakers
Sony Xplod CDX-GT540UI

planning on buying Alpine 51/4 speakers for the doors, then I will need a box and amp, and possibly a new deck?

What would be a good amp to power the subs, a good box and would I need a new deck?

Any help/ advice would be greatly appreciated, thanks.
alright, but what are the exact specifications that i need to look for? like how many channels or what wattage and how many ohms?
 
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