I just got an iPhone 4 passed down to me. A few questions?

Title says it. I need to know if I can use a prepaid phone card on a Verizon iPhone 4. I have Verizon, but I would rarer use a prepaid card. I need minutes mostly, and some messaging would help but I don't need web. Also, what's the difference between 2g and 3G? How does 3G work?i read somewhere that you can't use prepaid cards on the 4, is it true?