Ok... I was trying to code it all up from scratch and make new slices of your images but... I give up (sorry) because I have no idea what the site is for. And I need that to answer the following stuff about the boxes:
1. are they going to extend downwards?
2. are they fluid-width (meaning one set of background images for different box-widths)?
3. can I or can't I remove the transparency of the rounded corners. If you have a changing background, other than white, you need the transparency... if it will stay white for the whole time you're going to use this layout, you can remove the transparency of the corners (which is where all the problem is coming from in the first place...)
If you have a plain white background for the page, I suggest you open your original image files and disable the transparency. That was the initial problem. So if you make the corners a solid color outside the rounded outline (except for the top-right-star background), you won't see the extended borders anymore and you can revert back to your original code.
Here's how I would do it if I can't remove the transparency: