sounds good man, fits into the mix quite well, guitar is a bit loud though, tone sounds great.
Mastering is usually the process of preparing multiple songs for a master, usually a CD, and is done by a mastering engineer, not the producer. It's the post production of mixing multiple tracks, after they've already been mixed individually, to fit together volume, tone, etc-wise. If this is what you're talking about there's a nice, simple write up at http://www.tweakheadz.com/mastering_your_audio.htm

If you're meaning mixing, I'd highly recommend http://www.soundonsound.com/sos/dec09/articles/metalII.htm
However, to answer your finally question, there isn't much mixing you can do with your single guitar track over an mp3. I think you've got an appropriate volume for a cover, it's always clear what you're playing but not so loud that you cant follow along with the original song.