qemu -m 512 -boot d -cdrom archlinux-2010.05-netinstall-x86_64.iso
This loads the image, and I can select to boot Arch Linux the normal way, but it will give me an error that I have to use a 64-bit machine to run this live CD. Now, I tried to get a list of CPUs QEMU can emulate for me, and ended up with this output:
axle@z370l:~/Downloads$ qemu -cpu ? x86 [n270] x86 [athlon] x86 [pentium3] x86 [pentium2] x86 [pentium] x86  x86 [coreduo] x86 [kvm32] x86 [qemu32] x86 [kvm64] x86 [core2duo] x86 [phenom] x86 [qemu64] x86 [host]
So, how do I make QEMU emulate a 64-bit CPU?