Using auto margin in CSS but want a minimum margin

It might take another div to accomplish this task. I will refer to the div you are talking about as the #content div. I know that yours will have a bit more css. This is just my example of the margin in question. Next we would put the #content div in a div we will call #container. We will set the margins to auto in this div as well. The added aspect will be that we will add padding (right and left) to the #container div.

#content {
    margin: auto;
}

#container {
    padding-right: 5px;
    padding-left: 5px;
}

I think that this would achieve what you are looking for. Keep in mind a min-width for the #content div and it could work nicely.