Is there any significant resource usage difference between the two? My average cpu usage by XFree86 is 3-26% under a normal load... can this be signficantly improved by switching to xorg.

no. they are 99.9% identical code base. you use less cpu by not using KDE or other insulting bloatware.

yea xorg has for many years incorporated the xfree code and not done much else
now that stops with the introduction of the new xfree liscence -- the license most likely a reaction to behind the sceens tries to move xorg to the forefront.. because people were unhappy with xfree's more closed developement style

so the two are just now diverging
interesting to see how it turns out
xorg will no doubt have lots of work done to advantage the buisnesses that support the consortium
but continue without the help of xfree
xfree will plod on like always for later when xorg goes proprietary again and tries to sue everyone like those big computer companies always do

what exactly are xorg and xfree86? if not guis like gnome or kde, what are they?

Check out the wiki: http://wiki.linuxquestions.org/wiki/X_Window_System