You will want to go w/a 600 watt psi just so there is some head room for upgrades etc. I would average normal usage of the computer would be about 520w so a bit of headroom is always nice.
If you get a 600w make sure it's at least 80plus bronze. http://www.newegg.com/Product/Product.aspx?Item=N82E16817139028
This would do nicely.