Along with this there is a thread here about how to change out the stock speakers to a full audio system speaker 3-way, sub, mid, tweeter, set up speaker. Its cheaper than the sub and less hasle, and have read that is sounds close to the same with better clearity.
There Polk audio DX7's 5X7 and run around 150-170 a pair. Good luck finding them in a local store though. My area doesn't have them.