I remember reading the session fall back was no longer available for Ubuntu - maybe I mis-read or misunderstood.
Today I installed Ubuntu 14.04 beta 2 on an old Dell laptop; after I validated that all worked I started to look for ways to make Unity more to my liking and was not at all successful.
I found Gnome Session Fallback using Synaptic; installed it; and now extremely happy.
Dell Inspiron 6000
2 GB memory
Intel® Celeron(R) M processor 1.50GHz
Intel® 915GM x86/MMX/SSE2
disk 37.1 GB