Your stereo is probably putting out 15 watts per channel in reality. A 4- channel amp pushing 50-60 watts per channel rms @ 4 ohms would work well for you.
I just installed one of these in my Chevy
http://www.sonicelectronix.com/item_...s-GLX60.4.html
Fits perfect under the driver seat. Inexpensive but still sounds good. It will give you better sound and more volume without distortion. Better bass.....maybe. That requires a sub for good bass. I don't know how well those Undercover speakers will produce bass. More power will definitely help. Those are rated at 200 watts, so I'd bet RMS is around 100.
Stepping up to this:
http://www.sonicelectronix.com/item_...-ZRX600.4.html or this
http://www.sonicelectronix.com/item_...ZRX1000.4.html will get you a little more out of the undercover speakers.
If you have never installed an amp before, do some reading on
www.Crutchfield.com or on
www.SonicElectronix.com. They have some good articles.
Whatever you do, don't cheap out on the wiring kit. Good ones are expensive. Cheap ones have really crappy wire and can lead to amp failure.