The difference is night and day. The difference is being able to be fairly confident that your mix will sound the way you would expect it to given a wide range of playback systems. The difference is being able to hear your music and not something that kind of sort of sounds like what you might have recorded.
Once you can hear what you're really recording, your mixes will improve very quickly. I would say that the two most important things I have purchased in my short time recording have been monitors and a decent soundcard. Recording is no fun when you can't reproduce the sounds your making, and mixing is no fun when you can't really hear what you recorded.
I can almost guarantee that if you purchased a pair of monitors and played back one of your mixes you'd be able to hear about a dozen ways in which the mix could instantly be improved.
Note that monitors are not necessarily flattering either. They don't always sound as good as high quality hi fi speakers for just listening to music. However, they will be consistant and make all those little imperfections stick out like sore thumbs.
Slackmaster 2000