We can't really answer your question without knowing what upgrades you may want to do in the future. Just as an observation, the 850w you chose would be good for a pretty massive gaming rig including two high power gpu's like the dual processor Titan. If you plan on upgrading in a SUBSTANTIAL manner, buy the 850. If you are planning to have a decent everyday gamer, browser, general purpose machine - 650w still leaves plenty of room for a very nice gpu like the GTX 780 or R9 290.
Mark