Thinking more...

If you wanna try LXDE in Ubuntu, just add this source:

deb http://ppa.launchpad.net/lxde/ubuntu/ hardy main universe multiverse restricted

, then install lxde and lxsession (sudo apt-get install lxde lxsession).
After installation, logout. Switch session to LXDE, then login again.

LXDE is really fast, but pcmanfm don't support smb:// and network:// protocols, so I just cannot switch to LXDE...

Additional, there is a source:

deb http://people.linux.org.tw/~pcman/ubuntu/ ./ 

in LXDE's download page.