There's nothing wrong with having too much power available, but if you're only using a fraction of the unit's rated output then you won't hit the efficiency sweet spots. Ultimately it's negligible to electricity use and cost, but some units can have dodgy power delivery (like lots of ripple or spikes) if you're not near the efficiency range.
It's rare though, so don't worry, and it's not like your system is drawing a mere 20W or anything
I'd say at full tilt you'd draw about 350W, so that's 54% load for the 650W unit and 41% load for the 850W - the 650W would actually be running more efficiently than the 850W in this case. But as I said it's negligible in the real world. In either case you have more than enough juice available to clock the pants of both CPU and GPU (case and cooling permitting). 850W would be good for SLI in the future, but even with the super-cool 970s I wouldn't SLI in a Phenom.
Thanks for clarifying, SLI isn't a plan for this PC so no worries there.