Howdy,
My advice is to go with a 4-channel amp as you already mentioned. Axe the rear speaker idea, all they do IMO is screw up your imagining and front stage. Use two of the amps channels for the best componet speakers you can afford. Since you are not into obsence bass, use the remaining two channels of the amp to power a single 8" or 10" sub.
It will not take up much room and the sound quality will be much better than the 4 main speakers you speak of.
PS I have Inifinity Perfect 6.1s in the front of my 02 and I love them.