Lubuntu is a fantastic choice, I install it on all of my friends' old, crapped up, virus infested Windows XP computers and it has them running like new. The interface requires almost no adjusting to coming from XP, much less so than unity or xfce or even windows 8 for that matter. Throwing away old but functioning hardware should be a crime.
Yeah, except that the cost of power to run those machines can be significant. The most obvious difference would be reducing consumption by replacing a CRT monitor with an LCD. That should have minor effect on Ubuntu ... today ... maybe. But I do remember fighting to get a monitor configured a couple of years ago (before I moved to a Mac).
Are modern desktops more efficient than 10 years ago? Hard for me to say although Intel has a paper that says that Energy Star and other measures make a big difference.
Probably the easiest thing to do would be to swap the old desktop for a modern laptop. You'd gain all kinds of energy savings - LCD screen, automatic sleeping, etc.
