Although I have a very modern and up-to-date computer with 4 gigs of RAM, I do like the desktop environment of LXDE, which has a very light footprint. I have customised it to my needs and preference, with a top panel for everything and cairo-dock in the bottom panel place.
LXDE stands for "Lightweight X11 Desktop Environment" and its a desktop environment which is lightweight and fast. Its ideal for older machines with limited RAM, and has a lighter footprint than XFCE. Its available in the repos and can be installed when you install the main operating system.
You can find further information about it at the LXDE wiki and they also have a forum too. The mailing list is very good and helpful and has good archives for scanning back in previous posts.
Moving on to 'cairo-dock' this is a dynamic panel which sits at the bottom of the screen and has icons for all the programmes that you have running. If you're using 'xchat' and someone mentions your nick then the icon spins to attract your attention! Whichever programme you are currently using has a small outline box drawn round the icon to highlight it. It is easier to go direct to the programme you want to using the mouse and cairo-dock than going to the desktop and then moving through all the windows there of your programmes. Its installable from the repos and has its home page at GLX-dock where it has a good set of documentation, a wiki and a forum too. The Debian Wheezy has version 3.0.0 but there is a more recent version in the Debian Sid repo of 3.1.1.
I've tried the upgrade of cairo-dock to that as used from the Sid repo, and it fails but I forget how, which isn't very helpful really.