See: http://jsfiddle.net/b2BpB/1/
Q: How can you make box1 and box3 align to the top of the parent div boxContainer?
boxContainer
#boxContainerContai
As others have said, vertical-align: top is your friend.
vertical-align: top
As a bonus here is a forked fiddle with added enhancements that make it work in Internet Explorer 6 and Internet Explorer 7 too ;)