Re: Cover letters
Mentioning minimal experience is a No-No... you should be applying to jobs where you've determined that you either have the skills and qualifications or at least the aptitude to work.
If experience is so important, then let the firm screen you out after looking at your resume... don't make them short-circuit you from their selection process at step 1 (cover letter) - at least have them proceed to step 2 to glimpse over the resume'.
I agree with Lussi's suggestions... you will need to reword the cover letter to embellish it with some more action verbs. Also when you note your jobs in the second para... make sure that you're able to answer the "so what?" for the person who's reading your resume. This is to say that you need to map some of your experiences in jobs1,2,3 etc. to the requirements of this job and make this very explicit in the cover letter.
Lastly, in terms of the structure, you need to start general in terms of your qualifications, then be specific for a couple of paragraphs highlighting the relevance of your experience, and then be general again in asserting your suitability for the job.
Hope this helps.