If you get the grub "prompt" and can't enter anything, it means;
a) grub stage1 is in the MBR, and (usually)
b) it can't find the stage file/menu.lst ...
It's not really a prompt, it's a (deliberate) hard error message loop.
chroot into your disk install, cd into the /boot/grub. and run this
The 64-bit Arch runs fine here on both dual and quad core 2 ...