Ok for all those who spend hours and hours on blogs here and expertzzz....why cant you spend 10 minutes and find all needed info.
Let me go off.....

case 1

If I come to your site and just before I upload movie or Music file I click on "I agree" which basicaly says that I am the owner or have copyright....

So for you as the owner, you`ve got nothing to worry. You ARE NOT RESPOSIBLE for third party uploads. Plus you asked me if I have copyright before upload....so sleep thigt.
I see more really cant look for the title of the law explaining above mentioned.
No one can sue youtube for the very same reason. They do try but they end up paying youtube`s attorney fees for nothing....

Paste or edit below to fit your needs:
xxxxxxxxxxxxxxxx
Digital Millennium Copyright Act Notice

It is our policy to respond to notices of alleged infringement that comply with the Digital Millennium Copyright Act (the text of which can be found at the U.S. Copyright Office Web Site) and other applicable intellectual property laws, which may include removing or disabling access to material claimed to be the subject of infringing activity. If we remove or disable access to comply with the Digital Millennium Copyright Act, we will make a good-faith attempt to contact the owner of the material so that they may make a counter notification pursuant to sections 512(g)(2) and (3) of that Act.

Please refer to the following detailed instructions which must be followed to protect your rights under the Digital Millennium Copyright Act.

Infringement Notification
To file a notice of infringement with us, you must provide a written communication (by email or regular mail) that sets forth the items specified below.

Please note that you will be liable for damages (including costs and attorneys' fees) if you materially misrepresent that material on our site is infringing your copyrights. Accordingly, if you are not sure whether certain material of yours is protected by copyright laws, we suggest that you first contact an attorney.

Your communication must include substantially the following:

1. A physical or electronic signature of a person authorized to act on behalf of the owner of an exclusive right that is allegedly infringed.

2. Identification of the copyrighted work claimed to have been infringed, or, if multiple copyrighted works at a single online site are covered by a single notification, a representative list of such works at that site.

3. Identification of the material that is claimed to be infringing or to be the subject of infringing activity and that is to be removed or access to which is to be disabled, and information reasonably sufficient to permit us to locate the material.

4. Information reasonably sufficient to permit us to contact the complaining party, such as an address, telephone number, and, if available, an electronic mail address at which the complaining party may be contacted.

5. The following statement: "I have a good faith belief that use of the material in the manner complained of is not authorized by the copyright owner, its agent, or the law."

6. The following statement: "I swear, under penalty of perjury, that the information in the notification is accurate, and that I am the copyright owner or am authorized to act on behalf of the owner of an exclusive right that is allegedly infringed."

contact info: so and so
xxxxxxxxxxxxxxxx

For all those who have adult web site there is 2257 right? Well it doesnt apply simply b/c its uploaded by third party. If you do make porno muvies and take picture of yourself and other people then, and just then yo uneed to comply by 2257.
If you need a hand paste/ edit bellow to fit your needs:
xxxxxxxxxxxxxxxxxxxxxx
Section 2257 Exemption Statement
All persons depicted on this Website were 18 or older when the images were created.

Exemption Statement - Content Produced by Operators:
With regard to all visual depictions appearing on this website, for which the operators of this website are "producers," such depictions are exempt from the provisions of 18 U.S.C. §2257 and 28 C.F.R. 75 because:

they do not portray actual sexually explicit conduct as specifically defined in 18 U.S.C §2256 (2) (A)(i)-(iv);
they do not portray depictions of the genitals or pubic area created after July 27, 2006;
they do not portray simulated sexually explicit activity created after the effective date of Title 18 U.S.C. §2257A; or,
they were created prior to July 3, 1995.
Exemption Statement – Content Produced by Third Parties:

<<bellow applies to everyone having music or video files>>>

The operators of this website are not the "producers" of any depictions of any actual or simulated sexually explicit conduct or any othercontent. To the extent that any such content appears on this site, the operators' involvement with respect thereto is limited to the transmission, storage, retrieval, hosting and/or formatting of depictions posted by third party users, on areas of the website under the users' control. Pursuant to Title 18 U.S.C. §2257(h)(2)(B)(v) and 47 U.S.C. §230(c), the operators of this website reserve the right to delete content posted by users which operators deem to be indecent, obscene, defamatory or inconsistent with their policies and terms of service.

Questions or comments regarding this Exemption Statement should be addressed to: so and so
xxxxxxxxxxxxxxxxxxx

I thing in this case , the only thing to wory is the storage size in your server. And paying more for more storage.
You also can add following link for them to get informed so they cant say ; OH I didnt know

http://www.copyright.gov/

Ok I hope this helps.
Nurke