In this example, I use the actual anchor tags to put the borders in. No margins or padding is needed as the links are centered. Hover cursor over top links to see colour change. The H1 heading is resized with margin zeroed and required padding set. Font weight is set to normal. I've used an H1 as more importance is placed on this by search engines.
Minimize this page and alter the width. The boxes should be ok until the text inside the boxes gets forced out. In IE, you'll see the gap at the R/H side alter slightly, due to IE not being accurate in calculating percentages.