Depending on the value of your mountain bike, I would recommend that you upgrade the wheels next. I work in a bike shop, and everyone of us working there has put their clipless pedals on, and then new wheels. Most of us don't even ride the stock wheels, we just sell them and put the nicer wheels on. The difference in weight and durability is the most significant difference you can make on a good quality mountain bike. You can drop hundreds of grams by changing your wheel set, try to find another part that can drop hundreds of grams so quick?
