Start off with a head unit. I'd say get a McIntosh head unit and maybe a changer...say 12-disc?
Hook that up to a Dynaudio System 3-way speaker set with a phoenix gold amp.
Next, try a PPI PCX-2400 amp... with 2 Kicker Solo Baric S15L7's in an enclosed box.
...anyway, i'm just kiddin' ya pal... state your price range next time.
you don't necessarily have to get a new head unit for subs and an amp. i had my stock head unit hooked up to a hi/lo convertor and ran that for a while with my amp and subs, but after a while you dont want to do that anymore because it honestly sounds like crap (and i felt guilty about it).
I'd say get a head unit and then if you need extra bass, then subs+amp. if you think the head unit gives you good enough sound quality then stay like that or change some interior speakers.