I'd buy a real computer before I settled on LXDE. As far as I can tell, LXDE exists mainly to run on computers that were already considered crap around the time Windows XP came about.
Edit: Also, XFCE can be pretty obese depending on the distribution that is shipping it. Ubuntu's XFCE comes preloaded with a bunch of really godawful Unity crap and nonsense from GNOME 3.
Isn't the entire point of XFCE to be light and functional? Not many distributions strike that balance very well.
I tried out Fedora 18's spin of XFCE yesterday, and it's looking pretty good (there's still a few pieces of 4.9 left on the latest nightly, but you can upgrade those few packages and log otu and back in, and you can see what XFCE done right is looking like on a decent distribution.
Now, I did notice one odd thing. They don't ship the volume daemon running by default, but you can install that and add it to your start programs. Change the theme around a little bit, and you're good.