You can add a amp to the stock stereo pretty simply. Jaycar has everything you need.
Simply cut the speaker wires going to your front doors and install a line out converter -
http://www.jaycar.com.au/productView.asp?ID=AA0482Run the RCA to your new amp (mine is under the drivers seat) and then run speaker wires from your amp back to the original speaker wires. I used the jaycar kevlars -
http://www.jaycar.com.au/productView.asp?ID=CS2402 and an old response 2 chan amp. -
http://www.jaycar.com.au/productView.asp?ID=AA0450While your at Jaycar you may as well grab the wiring kit -
http://www.jaycar.com.au/productView.asp?ID=AA0442It took me a whole day to do but it was worth it. I'd recommend taking out the front seats whilst doing the install, it makes for a more pleasant experience. You should probably also get on Bens resomat group buy and do your doors when you are swapping out the speakers.