I use KDE

, on 5.8 SOHO. I trimmed it down quite a bit though:
- removed the bulk packages I don't use (kdegames, -toys, -network, -pim, -edu, perhaps some others as well)
- turned off animated stuff, and aRts (it's evil!!!)
- turned off all the services I don't use (cron, samba, cups).
I use KDE because I prefer some programs that use qt as well (amarok, kaffeine, kmix, kpdf), so a lot of the libs that would have to be loaded for these programs under another wm are probably already loaded for KDE. Here's a screenie of memory usage in KDE:

Note that this is with just one tab in firefox, amarok, kmix and yakuake running. If I need something lighter I usually switch to IceWM. I use it because I'm used to it (from VL 5.1) , not sure how well it performs compared to other WM's. When firefox starts to eat all my resources I usually just close it and open a new one.