a 57 is a fine mic, but not the best for capturing acoustic guitar.
i own 3 57's, one dates back to 1982, and is still working fine after 4 years of fulltime roadwork, it is a workhorse, and will serve well.
but it is not the best choice.
it is a dynamic mic, and you keep mentioning wanting a condensor.
this all boils down to two things:
1. how serious are you about recording?
2. how much can you afford for a 'decent' mic?