In that case, get good headphones first if you're not on a big budget and value portability. This subject has also been discussed so search and you shall find something useful .
5.1 sets aren't useful for 2.0 music. Most computer speaker sets also place emphasis on the bass and high so whatever music you make might sound great on the speaker set and absolutely horrendous on anything else.
Good headphones will be cheaper than studio monitors and portable, and the imaging will be better.
lol, dumb question, how do you know if its 5.1 or worse/greater? im lost! whats 2.0 music?