for a graphic and music site? My site will contain mainly music and galleries, so I wonder is 5 gb enough.

Depending. How much music & pics are you adding? Music shouldnt take much, because you dont upload it to your file manager, you just link the music into your html code..

Well, small blogs normally get 10-15 mb, and I barely used that much on geocities, even after I uploaded a ton of graphics and music clips, so I'd say that 5 gb is DEFINATELY enough.

are there any way to tell how much bandwidth does a certain site have even though you're not the webmaster of that site?

no, I don't think so.
If you send an email to the webmaster kindly asking him how much space/bandwidth he uses, than you may get lucky and get a reply.
I wouldn't mind telling how much I use.

Then can I ask how much bandwidth you use?
right now with all the stuff I have it will use up to 300mb of webspace. I upload the musics on my site.