You can use this to estimate it: http://www.extreme.outervision.com/p...ulatorlite.jsp
Based on some of the info you gave me, I only got it up to 260w. I'm fairly certain you'll be okay - the 400w minimum rating includes other factors like CPU, motherboard, memory and hard drives. Unless you went all out, you're probably under 350w.
From nvidia: "Lower power configurations are possible using lower total power system configurations (CPU, Storage, Memory)."