For 64 bit procesors first you must to edit /etc/rc.conf like:

[multilib]
Include = /etc/pacman.d/mirrorlist

upgrade your system:

# pacman -Syyu

and install gcc-multilib and its dependencies

pacman -S gcc-multilib gcc-libs-multilib binutils-multilib libtool-multilib lib32-glibc

With testing in Oracle VirtualBox first I must to make changes as it is described here:

In /etc/inittab change 

id:3:initdefault:

[...]

x:5:respawn:/usr/bin/xdm -nodaemon

To:

id:5:initdefault:

[...]

x:5:once:/bin/su - -- PREFERRED_USER -l -c '/usr/bin/startx </dev/null 2>&1' 

Then I have to switch run level through GRUB as it is described here:

In /boot/grub/menu.lst find the Arch section (the default is '# (0) Arch Linux'):

# (0)  Arch Linux
title  Arch Linux
root   (hd0,0)
kernel /vmlinuz-linux root=/dev/sda3 ro
initrd /initramfs-linux.img

Change the kernel line to:

kernel /vmlinuz-linux root=/dev/sda3 ro 5

  and then finally I should do as it is described here:

Edit /etc/inittab and find the line that looks like similar to this one (delete # sign)

x:5:respawn:/usr/sbin/gdm -nodaemon