First thing to do to lower memory consumption is to stop using applications that are memory hogs. The applications that I had to switch away from were Banshee, Evolution, Liferea, and Tomboy. The applications which I switched to respectively were Rhythmbox, Thunderbird, Google Reader and none (graduated from school so no longer need a note taking application).
Second thing to do is to make Firefox more friendly. In Preferences » General uncheck the Don't load tabs until selected checkbox and set the When Firefox starts option to Show my windows and tabs from last time. Whenever my computer gets slow, I close Firefox and re-open it which causes all tabs to not be loaded except for the one that I am currently using. If you have flash installed then the Flashblock plugin is necessary. Firefox tabs in the background that are using flash are very demanding and Flashblock helps you stay aware when Flash is in use. AdBlock will also help keep Firefox more resource friendly if you are desperate.
That is all the advice that I have for you. I have found the Unity 3D interface to be very usable on my older netbook (Intel Atom N270 with GMA 950). My computer does get slow at times but that is almost exclusively a Firefox problem but I tend to open up a lot of tabs so that is easily fixed.