Topspin serves are good for second serves. The swing speed should be the same as for the first serve, the main difference is the toss, it shouldn't be so front of your body as your first serve. This helps get more spin than power. Even if you want a powerful second serve, you should remember that power in second serve is useless if you don't get it in with over 90% chance. You should first concentrate on getting good spin and high serve % and when you have mastered this , then try to go for more powerful second serve by tossing a little bit more further.