It looks to me as though you may have already been playing with the embedded width for these boxes. That is indeed the way to go, but you haven't reduced them far enough.

The boxes have an internal width for their content, but are then made wider by adding padding and borders. Margins further increase the amount of space the boxes take up.

When the expanded widths of the boxes plus the margins between them exceeds the space available, the next will drop down to the next row, which is what you are experiencing.

So basically if you want to add spacing and borders you must make the boxes thinner to compensate - and bear in mind that you may need to take a special look at your site in IE6 and it tends to double the margins for floated boxes and generally gets its width calculations a little wrong.