A lot of questions about credit.. in the end, it is often about your FICO (Fair-Isaac) score. Go to http://www.myfico.com/ and do some reading.

They are trying to sell you regular reports - but there is a lot of interesting information there, and FICO will affect the rest of your consumer life, including how much interest you will be offered on car and home loans, etc.

Don't worry too much about upgrading if it will get you into a debt sinkhole...