1000 MB is fine and dandy for 13 bucks, but what is your bandwidth charge? If you're only getting 5 GB of transfer or something, a theme site will double that in a month...leaving you screwed with overcharges.

You are better off with having the artists host their themes and just posting screenshots and info into a database.

Also try to get a macintosh based hosting company and maybe you can strike a deal with advertising/hosting since it wouldn't be a high bandwidth site but would be high traffic, I'm sure they'd love the exposure.