the problem is that the content area does not have enough space available to fit the 659px wide gallery
The content wrapper is 900px wide
the Right column is 209px with 14px padding on top, right, bottom left making the actual width 209 + 14 + 14 = 237
900 - 237 = 663px
This leaves 663px for the content window. you have a 15 pixel padding on left leaving 648px of space to fit the gallery into.
If you remove the right and left padding from the content and right column, you will be able to fit the gallery.