I just wanted to followup on this since I finally figured it out and thought someone might need to know how to do it at some point.

The original code I used works for the most part. All I had to do was change the border-style: none; to border-style: double; for the plain classes and make the border color white. That way, the border is really being drawn all of the time for each box, thus preventing the size to change. On the mouseover, the color is changed and the border appears. Works well in IE and FireFox.