Are you on a really old release of RHEL 4? You have to go all the way back to 2004 for a GNU Tar release before 1.15 according to http://www.gnu.org/software/tar/#releases
I believe the Amanda compatibility page is primarily pointing out difficulties that tar has when you push it to do complete backup and recovery. Check the footnotes at http://wiki.zmanda.com/index.php/FAQ...-compatible%3F
. Nothing to stop you from using an older Tar, except that you will probably run into difficulties when it is pushed.
I wouldn't stoop to cp if I was concerned about doing a complete image backup and being able to recover from it. There are lots of tools designed for this purpose. Find one that suits you. Try googling "linux image backup" and take it from there.