If you start with the base install, you can update with pacman -Syu, then add XFCE with pacman -S xfce4 xorg. You have to generate your own xorg.conf file, or use hwd to make one. (I have this all written down on my little scratch pad on my desk.)

If you open /etc/pacman.conf, you can add this line at the bottom ...

Server = http://arch.os-zen.net/pkg/xfce-svn
then add the newest version of XFCE with pacman -S xfce4-svn xfce4-svn-extra.

The best place to look for this stuff is in the Arch wiki. It's very useful. I don't spend much time on the Arch forums though. I like it better here.