Q: Do I really need a cover letter?
A: Everyone who sends out a resume does! Even if the cover letter never "came up" in conversation or was not mentioned in an advertisement, it is expected that write one to submit with your resume. A well-designed cover letter gives you another chance to emphasize what you have to contribute to the company or organization. Do not give your potential employer a second to entertain the thought: "But how can this person help US?" Your cover letter will answer that question for them.
Q: Should I just list all of the places that I have worked for in the past?
A: I would only list points that directly correspond with action items expressed in the job posting. For example, if the job posting states that they are looking for someone who has experience handling a sales volume of over 20k annually, than one of the things that you should mention in the cover letter that while you were with Company X, you handled accounts 25k and/or higher.
Q: Should I include salary requirements in the cover letter?
A: Include salary requirements in your cover letter only if requested to do so. CareerBuilders.com and Monster.com have great tools where you can research salaries in your area.
If you are still not comfortable with producing your own cover letter, you can always leave it up to the professionals. Check out my website, and I can produce a creative and professional cover letter that you can present with your resume.