What fruhulda said.
And, you might want to create a new document template for your letterhead as well. Once it is laid out as you like, save this new template (let's call it ltrhead.template) into a folder called My Letterhead. The file and folder names are arbitrary.
Now, when you start Pages, your newly created Letterhead will appear in the Template Chooser under the new category: My Letterhead.
I normally test things before I post. This time, I tested after I had posted and logged out of the community. Arrrgh. What I posted works with the following update:
I discovered that Pages will only recognize your newly saved template if it is placed in the default template location, or a folder created under that hierarchy. And within the Template Chooser, it will only report a new category as My Template, and show templates you have added to this default location. If you placed an arbitrarily named sub-folder under My Template, the Template Chooser will not list it, but does show each template within the sub-folder. This is a substantial departure from what the Pages '09 help pages describe.
When you save a new template, the default location is:
<home directory>/Library/Application Support/iWork/Pages/Templates/My Templates/
You can have a named Template folder but it must be inside the Templates folder, outside the "My Templates" folder, a known bug.
Templates will open and work from anywhere but you will need to navigate to them and they will not appear in the Template Chooser.
btw It is best to recreate your letterhead using fonts and vector artwork if possible.
Failing that scan the graphical part at its highest possible resolution and combine it with text. For photo like images with graduated tones that is 300dpi, keeping in mind that if previously screened and printed that will cause moire patterns. If it is solid color artwork such as single colors, or black and white, scan it at at least 600dpi (1200 is better) and save it as a compressed cmyk or b&w tiff. JPEGs will blur the image and give fuzzy edges.