ehhh...idk about overclocking with that quality of PSU...but it does support xfire and I think that's enough to run at stock. Me personally I wouldn't settle for less than 850w quality brand but that is just so I can sleep at night. Thermaltakes aren't bad, but they aren't the highest quality PSU's...do you happen to know the actual manufacture for that model PSU from thermaltake? that could make a difference. You could use a wattage calculator to be certain, I think each card uses about 150 watts, and since the 2nd one probably wont get all that its supposed to maybe plan on an extra 100 or 150 watts on the PSU...how many HDD's and what kind?