Light and quick for a desktop/window manager, but be sure you're not afraid of tinkering with config files to customize it your way. Easy enough to figure out for an intermediate user. Adventurous beginners only.
One of the best window managers around - simple, blazing fast and easily configurable by text files. If you need to fit your GUI within 40 MB of used memory, then IceWm is the best choice directly on top of the X server. In combination with pure LXDE you may fit within 100 MB of used memory. However medium to advanced linux experience is needed to set it up and running properly.