Definitely get a tablet. They're much easier on the hands than a mouse is. I also use an Apple trackpad with my desktop computer, but I switch back to a mouse or my tablet when I'm doing something that requires high mouse precision (gaming, design software etc).
I use the mouse in my non-dominant hand at work when my RSI is bad. It was a little weird at first, but now I'm just as good at it as I am with my dominant hand. This one will depend on what you're doing with the mouse, but for ordinary computer use it should be fine.
I also wear wrist splints at night (this may sound weird, but it really helps - it was the only thing that cleared up my RSI last time it was really bad), and when I'm on the computer at work. They're no good for drawing because I need the wrist movement, but for clicking and keyboard work they're perfect for keeping my hands in a healthy position.