[hlfshell]

Xubuntu is your "lite" release for old hardware. It's shipped with XFCE instead of Unity so it doesn't require much to run. I believe Xubuntu 12.04 is released same-day as the 12.04 release.

[krakensden]

The latest Unity 2D is actually quite snappy on my machine, for what it's worth. More so than Unity, in fact.