Ubuntu 11.10 has taken the necessary step up updating from Gtk2 to Gtk3. Most of the system is noticeably slower then previous releases and requires a faster computer to run smoothly (does not mean you cannot run).
With your computer specs I would recommend that you run Ubuntu 10.10 (change the release on the dropbox Download Options) or running any Ubuntu alternatives with a lighter desktop environment, ie: xubuntu or lubuntu.
If you still think Ubuntu is the way and 11.10 is the release to go then you should be for sure using Unity2D instead of the default Unity3D.
There are some tweaks that you might do and get some very limited boost in performance, but those take more work than should and the difference in the end will barely be noticeable