LXDE would be good of course, although I personally prefer Xfce myself. I have a desktop box around here with 512 MB of RAM on it too, and I have PCLOS Phoenix (the Xfce version) plus Enlightenment on there (I've tried LXDE but I really don't care for it -- I'm too hooked on eye candy! LOL). It was from that same computer that I created a custom ISO using MyLiveCD, which I installed onto a 4GB flash drive I always carry with me. The PCLOS installation I have on that flash drive runs just fine on my wife's laptop, which also has 512 MB of RAM (she insists on using Windows XP on it, which is installed to the hard drive -- I still have yet to convert her to Linux LOL).
On only half a gig of RAM, I would concur with what everyone else on here has said -- KDE will be S-L-O-W on that thing. GNOME would probably run at a comfortable speed, but I'd really recommend going with one of the lighter versions, such as Xfce, LXDE, Openbox, or Enlightenment.