I don't run Amithlon anymore, so I can't help you there. However, I do have a triple-boot: Win7, Linux and AROS. I typically install AROS last because it uses GRUB 2 (I think) and sets up the correct boots for Win7 and AROS. I typically have to save the grub information for Linux and then insert it into the AROS grub to get Linux to boot.
Don't know if there's a better way or not... probably.
