Ruby Version Manager (rvm) is a shell function that lets you easily switch between versions of ruby (and their corresponding gems). These instructions walk you through installing rvm, ruby, and rails on linux for a single-user setup (ﬁles kept in ~/.rvm). Following these instructions as root should set everything up for a multi-user installation.
With a rooted phone, you can use ROM Manager (from the android market) to install Clockworkmod and backup your phone. Unfortunately, with the Samsung Fascinate, you have to do some mucking around in the command line before Clockworkmod can install itself.