For a long stay visa you need to have health insurance that will cover you in France, your ordinary insurance in the US does not cover this. Since you are not married I'm not sure whether or not you can be added as a dependent on your boyfriend's health care (someone will have to confirm).
If you do buy expat health insurance and get a long stay visa, you might also want to look into registering your relationship legally in France - it's called PACs and it's like a civil partership. After one year of being PACsed you can normally get a residency permit with work rights. Without being PACsed or married you won't be allowed to work unless you go through the process of an employer sponsoring you (quite difficult) and returning home to apply for a work visa.
If you do buy expat health insurance and get a long stay visa, you might also want to look into registering your relationship legally in France - it's called PACs and it's like a civil partership. After one year of being PACsed you can normally get a residency permit with work rights. Without being PACsed or married you won't be allowed to work unless you go through the process of an employer sponsoring you (quite difficult) and returning home to apply for a work visa.