the only way to truly have it load only once is to use IFRAMES. Make the main page with the flash header, and an iframe beneath it, then you can load different pages into the iframe without changing the main url. However i could not suggest this method due to all the bad things I hear about the iframes. Theres also something called a UFRAME which I read is more dynamic in terms of height and width...

look on my previous post, i suggested 3 other methods to accomplish similar effects too