When you sync the songs, you copy them to the itouch, so you could delete the songs from your HDD once you copied them once, but I wouldn't recomend that as you can't copy songs from your iphone to your HDD and if anything happens to your iphone or itouch, you can't restore it without a second copy of the songs.
The term "sync" replaces "copy" when it comes to devices like that. If this works i be lucky connect your device and go to my computer and click on your device. To show its actual files click on tools> folder options> view tab then select "show hidden files/folders" that should do it