Do you hate the stock CD player and controls?
If not, the best thing to do is get an adaptor for the main double DIN head unit which will interface with an aftermarket amplifier. Get yourself some nice after market speakers, a small 8" sub and a nice 5 channel amp with built in xover controls and perhaps some EQ functions.
The adaptor will allow you to keep your stock head unit and steering wheel controls but upgrade the sound significantly by getting better speakers and good amplification.
I would expect to pay around $1000-$1300 for all this to be done right by PROS.
www.onlinecarstereo.com is a good source for car audio stuff if you wish to tackle this yourself which I would recommend if you are handy. Car audio stuff isn't rocket science and if you can read a manual you can do it.
YOU CAN DO IT!! 
Good luck and get some good tunes going in the RAV4!!!
