What is the difference between filing a form 1120S and a Schedule C? Where and how does the taxpayer benefit? If the taxpayer formed an LLC in 2004 and has always filed using a schedule C can he switch to filing an 1120S?

The LLC is a business entity, not a tax entity. When the LLC is registered, the registrants choose how they wish to file their taxes. So if you have been filing Schedule C, you can continue to do so.

Now, there ARE advantages of filing as a subchapter S corporation (Form 1120S) versus a Schedule C, but there are disadvantages as well. It is much too complicated to try to explain on this kind of forum.

Whole books have been written about this subject, so I suggest you find one at your local library and read it.

Or you can schedule a meeting with a local CPA and have him/her explain the pros and cons of setting up a S corporation versus retaining your LLC and continuing to report under Schedule C.

The intended benefit of an S corporation is to reduce employment taxes by paying a salary lower than the total earnings. The primary cost is that your tax filings get more complicated, especially employment tax filings.

The only way you can switch to filing a 1120S is if you form a new entity. Right now you are considered a disregarded entity and are the sole member of the LLC. When you setup your LLC in 2004 you had a choice to be treated as a Corp or as a partnership for tax reporting purposes, since you are the sole member, IRS gives you the discretion to file as a disregarded entity and report inc/exp on schedule C

Look here for an example of what the other person said in regard to saving monies on employment taxes, there is a good diagram. S-Corporations vs. LLC: Income Tax Savings Benefits My Money Blog
I stumbled onto this site in need of AC repair help, I am an enrolled agent and have been doing tax for more years than I care to list :P

If my S. Corp filed a 1120s do I need to file a Schedule C and E for my personal taxes

Not unless you are running a separate business that has the S corporation as a customer or client. You should get a Schedule K-1 that reflect the income that the S corporation distributed to you which is reported on Schedule E Page 2.