A good rule is to figure out what your hardware is going to need and then add at least 25%. If you buy a PSU that just barely meets the minimum wattage it'll burn out sooner and run much more inefficiently than one that doesn't have to work as hard. You also want to leave a bit of room for adding hardware later. I usually figure out what I need and then add 50%. I learned the hard way that skimping on the PSU will come back to haunt you down the road. I would say go with one that's at least 800 watts and preferably 1000.