If you use WP or other WYSIWYG type editor - you can create a page or post there and copy the code to your FBML page (remove the head and body coding at the beginning and end of the HTML code to use on Facebook).
This will work most of the time. Also if you are a graphic person you can create your "page" in a graphics program, save as an image file and post to your FBML tab with HTML code.
The Boxes tab is supposed to disappear soon - Facebook update will be removing it. All the items you currently have on your Boxes tab should be moved to their own Static FBML tab or to your sidebar or they will be lost when the update is completed.
If you want to PM me with more details I might have an idea or two or be able to steer you to someone who does