If you're going for two 10" subs I would follow Dizzie's advice, and look at running some amplified components up front, otherwise you are going to have a very rear based sound stage.
Definitely go for some replacement rears, but I would look at some straight forward custom fit replacements and power them from the head unit.
All middle of the road manufacturers (alpine, Kenwood, Pioneer etc) produce a range of custom fits, and your nearest decent audio shop will carry a bible (HUGE fitting guide) for each manufacturer and will be able to tell you which model will drop straight in.