Any chance recommending an appropriate microphone?
You can use your current microphone via the iMic
<http://store.apple.com/uk/product/TX532ZM/A?fnode=MTY1NDA0MA&mco=MTI5ODU0NzQ&s= topSellers>
Which is better USB or mic with amp?
USB is usually more convenient since a pre-amp needs power. The sound quality depends entirely on the microphone. There are some high-qualty USB microphones, such as:
<http://store.apple.com/uk/product/TF238LL/A?fnode=MTY1NDA0MA&mco=MTY4MzcwNzA&s= topSellers>
There are also USB pre-amps for high-quality XLR microphones
<http://store.apple.com/uk/product/TV233LL/A?fnode=MTY1NDA0MA&mco=MTEwOTA4MjA>
There are also low cost USB microphones and headsets.