Honestly do not bother with rear speakers just yet, unless you spend most of your time in the back seat you would want the sound stage right in front of you as much as possible.
Put your money towards a great set of splits and a decent headunit. Keep your existing amp (post details of it btw) and add a subwoofer that would compliment the amp that you have already. Bridging two channels of the amplifer to run the subwoofer would give you around 100-120RMS, so use a sub that is efficient at this input. Something like a JLW0 which operates happily at 125RMS ($172).
Run the remaining two channels of the amp to your splits and you are a happy chappy.
NB: what I wrote above is based on a small budget, but it will do very nicely.
Read up on this, especially Chapter 5:
http://www.decware.com/secrets.htm