I had the same problem the first time I tried to install 8.04, same mobo, all SATA drives.
what fixed the problem for me was to add irqpoll to the kernel
paramiters.
press ESC at the boot screen/press "e" to edit the line/ add "--irqpoll" at the end of the kernel paramiters/ press "b" to boot.
here is my config from /boot/grub/menu.lst
## ## End Default Options ##
title Ubuntu 8.04, kernel 2.6.24-16-generic
root (hd0,2)/ubuntu/disks
kernel /boot/vmlinuz-2.6.24-16-generic root=UUID=FA8A65868A653FED loop=/ubuntu/disks/root.disk ro quiet splash irqpoll
initrd /boot/initrd.img-2.6.24-16-generic
seems the SATA controller chip does not correctly identify itself, this fixed the problem for me, I have been able to install 32 and 64 bit versions of 8.04 on the system useing this method.