XFCE is available as a PBI, as is Enlightenment. Gnome and KDE 3 are, too.
I have to say though that KDE4.2 is not overly resource-hungry. I have it here on an old box with 2 x Athlon 3000 MP CPUs, 1.5 GB of RAM, 20 GB hd and nVidia Ti5200 graphics card.
And since PC-BSD is built with KDE at its heart, you will never get rid of KDE completely, even if you run another DE.