you probably won't see a large improvement in performance
like jaguarskx said, you need either a ssd or a high-speed (10k rpm+) drive to score above a 5.9 in the WPI. Don't worry about your WPI score too much (it doesn't really mean much).
i have both a 10k drive and a 7200 rpm drive and the performance difference between the two is almost impossible to see...
i'd recommend sticking with your current drive