Well since I assume the old domain is no longer working, first you need to add your new domain name and make it a mirror of the old domain so you can access your information using the new domain. You will also need to make a new mysql hostmask using the new doman and update your wp configuration information to point to the new mysql hostmask. Then log into your wp admin and go to general settings. You want to update the wordpress address and site address to reflect the new domain. Next edit the domain name and make it fully hosted. Make sure to use the same user that you chose for the old domain (just makes things easier...). Now, you need to rename folders which is easily accomplished if you log into ftp. First delete the new domain folder and then rename the old domain folder to what the new one was. That should take care of it as far as WP is concerned.
For your forum and gallery it might be a bit different. You would need to update any configuration settings to use the new domain and to also update any hostmasks that were referencing the old domain name.