I actually think you're just shy of what you'd need. You can feel free to ask around or wait to see if anyone with any first-hand experience can help you, but according to Gigabyte's website it says you need 500w.
Power requirement 500W(with two 6-pin external power connectors)
taken from:
http://www.gigabyte.com/products/product-page.aspx?pid=4795#sp.