The command you want is:
pacman -S xfce4 xfce4-goodies gtk2-themes-collection esound
Read about GDM's: http://wiki.archlinux.org/index.php/GDM for Slim: http://wiki.archlinux.org/index.php/SLIM
If you want SLIM (instead of GDM) to Login, then run this command:
pacman -S slim slim-themes
.... and then add "exec startxfce4" to you ~/.xinitrc file:
(As a regular user..... NOT AS ROOT)
nano ~/.xinitrc
then add this to the empty file:
#!/bin/sh
#
# ~/.xinitrc
#
# Executed by startx (run your window manager from here)
#
exec startxfce4
Save with "ctrl + o" "enter" "ctrl + x"
Next you will have to edit you /etc/inittab (in root), so follow this part: http://wiki.archlinux.org/index.php/GDM#Inittab_Method