If you have jobs prior to 2006 that are directly relevant to the jobs you're applying for, it is worth listing them on your resume, even briefly. If they don't relate at all, I don't think you need to go back any farther than your last two, since it takes you almost 10 years back.
I wouldn't advise specifically saying in your letter that you were a stay at home mom either. Though it's not legal to discriminate based on that, people will find ways around it and to declare another candidate a "better fit" than you. You are better off simply stating that you are looking to return to the workforce, without giving a reason as to why you left it in the first place.
It is recommended by many HR professionals, and preferred by many employers, that you keep your resume to one page. So, fit as much as you can, but try not to go onto a second page.
Also, if you have volunteer work during the employment gap that could directly translate into on-the-job skills, you can list those on your resume as well. For example, if you're an accountant, and you served as the PTA Treasurer, you can list that.