Ultimately it will depend on the quality of the microphone you use, mate, if its a cheap mic, it will sound cheap. I had the strange notion you were going to just use the on board mp3 recorder mic! heaven forbid it!
most mp3 recorders nowadays have very good recording qualities which is excellent for a mic-input but not an onboard mic. find yourself a phantom powered mic and see if that works (i dont think you will get much response from a mic without phanton power – though other people are welcome to correct me)
Mods, can we move this thread since it isnt Premire Pro related directly.
All the best mate!
David Nagel