2,4-channel amp?

hey guys,
i've got a couple questions about my system. First of all, i've got an aiwa reciever, a pair of 300 w 6-1/2" on my front doors, and a pair of legacy 600 w each 6x9s in my back. I really love thumping sounds of the bass and it gives me chills when i can hear someones bass 4 cars next to me, but I don't have that. I was wondering if an amp will do the trick? Whats the difference between the 2 and 4 channel amps? How many Watts should i get on the amp? Will I need a Subwoofer to get that kind of bass? If I do get an amp, will crossovers make it sound better?Sorry for so many questions. Oh yeah one more thing. How can I get my front doors to stop rattling? I would love any input. You guys are great. I love this site. Peace Jon