The guys on this forum are extremely knowledgeable, and you'll notice that they'll all say the same thing: replacing speakers only will not yield any real improvements (especially for base stereo).
You'll need to amp the system, but because the base stereo has a built-in EQ, you'll also need to either:
a) get an EQ flattening device (MS-8 or some other device)
b) re-code your HU to hi-fi (this will force you to use it with an amp, and the outputs are no longer sufficient to power speakers directly
c) change your HU (this is the route I ended up doing, so that I could flatten the EQ and add new functionality that wasn't present in my system: Bluetooth phone/audio, iPod, GPS)
There's tons of info here if you search. Good luck!