If you're really concerned about RSI, there are some alternatives to the conventional mouse. Some specialist manufacturers now produce upright mice, where the mouse resembles an old-fashioned joystick with your hand gripping onto the vertical shaft. This position, with your hand and wrist held vertically, is supposed to be more comfortable for long-term use. You could also try a trackball. A lot of discomfort caused by the mouse use is a result of the regular and unnatural movements of the wrist. With a trackball, your fingers do all the work (or your thumb if you use one of those models where the ball sits under that digit).
In my experience, trackballs do take getting used to, and to be perfectly honest I've never got as fast or efficient with a trackball as I am with a mouse - and you can forget trying to play fast-paced action or RTS games with the thing. They do, however, keep your hand in a comfortable and natural position, and put less pressure on the wrists. Logitech's Marble Mouse is the best known of the old-school trackballs, but the Trackman Wheel and Cordless Trackman wheel are excellent thumb-based alternatives. Prices range from £15 to £50.
Finally, don't forget about graphics tablets and keyboards with built-in touchpads. Tablets are often thought of as design tools, but they can provide a more comfortable and efficient general purpose mouse replacement once you get used to them. Most of us have got used to touchpads on our notebooks, meanwhile, so using one for day to day work won't be a huge shift, and they put a lot less pressure on the wrist than the average mouse.