The below link will tell you a Graphics Card's recommend power requirement.http://www.realhardtechx.com/index_archivos/Page362.htm
You'll notice that that link recommends a PSU 100 watts more powerful than what Nvidia recommends on their website. This is because the manufacturer usually states the minimum power required
to run the car,. A GPU at max load draws considerably more power.
Your PSU should be fine. Just don't connect a tonne of hard drives simultaneously.