On a sheer whim I installed Scientific Linux 5.4. I have an old test box I like to experiment on. I was amazed at the speed and responsiveness of the system.
The test system uses an old Trident chipset which works superb with this installation.
Specifically, Gnome in it's older rendition runs very responsive. The 2.6.18 kernel with it's backports and such impresses. I know it's Redhat recompiled but still I wish many other distributions would stop trying to include the most up to date software.
At anyrate, if you haven't tried it do so.
P3 1.1Ghz Celeron / 384MB ram