A rice rocket is a car that, first of all, is not meant to go fast in the first place (i.e. Civic, Teggies, etc...) and 'modding' them with visuals that they think make it look better. This is to mock a performance vehicle, when really, it is just a slow car with visuals to make it look sporty (i.e. Coffee cup ****ler, big spoiler, underbody lights, etc...).
A rice rocket, is essentially, a "poser" car. It is trying to mock the visual aspects of a sporty look to make the car appear high performance, when really, it's all looks. Appearance vs. Performance, a performance car isn't rice, a car that is just appearance and no performance is a rice rocket.
Oh, and about that "what if" question presented earlier in this thread about a 650HP civic, here's one that is 500HP in a video, download at this link (right click and save as):
http://wam.umd.edu/~ryalavi/99VR4vsGSRTurbo.mpg