Need some gallery advice

i implemented a xml gallery for a friends site.
basically a grid design with the thumbnails just laid out. 3 columns of thumbs.
and on the right is where the main image is shown…

however 3 columns is not enough, he has more images. And since he wanted the site in a restricted area of a popup window, theres really no room for more.

Let’s hear some suggestions on how i could redesign the gallery,
Keep in mind that he likes the grid layout. So something that lets you see the thumbs laid out.