First of all Check the box on ipod so that your music will not automatically sync.
Then download all of your music from your computer to your ipod. Then install a program called iDump. iDump lets you download music back from your ipod. So try downloading a song or two from your ipod to see if it works perfectly. If it works then you can delete music from your computer.
Also, it would be a good idea to get an external hard drive. Hard drive prices have dropped dramatically in last few months. I think everyone should try to get one.