Shirt and trousers for stage 2, tie and suit jacket not really necessary as you're just doing computer tests not meeting anybody. Then for stage 3 (interview) definitely suit and tie.
Read the nats interview thread for tips on stage 2 prep, if you want to practice you can purchase skytest software online, otherwise trust in your natural ability on the day. If you don't pass first time, reapply a couple of years later, many don't get it first attempt.