Starmap :
I'm giving my brother the gtx 670 and get two gtx 660, and Sli , so I better have extra power
I put 850 watters in almost every 670 build if the MoBo is capable of SLI, 750 watters go in the non OC'd builds....650's go in single GPU builds:
1. Most of my builds use non reference factory overclocked cards ... the 670s can break the 200 watt power draw barrier when highly OC'd.
2. I's not about being able to provide 750 watts with a 650 watt power draw and calling it a day. If ya read PSU reviews in detail, you will see that PSU efficiency peaks at 50% load. More efficiency means less noise and less heat.
3. If the goal in an enthusiast build is to hit the highest stable OC. Voltage instability and ripple generally increase the closer ya get to that rated load. Both have negative effects on the stability of your OC
4. The cost difference between 750 and 850 is oft negligible.
Here's a recent video editing / gaming build where user's goal was 5.0 Ghz stable OC.
http://extreme.outervision.com/psucalculatorlite.jsp
High End Desktop
I7-2700k @ 5.0 Ghz, 90 % TDP
Twin 670 Asus TOP cards @ 1280 Mhz (add 40 watts per card)
4 RAM sticks
1 HD
1 SSD
5 USB
1 Firewire
Card Reader
4 LED / 6 Regular 140mm Fans (Corsair 500R)
90% load
15% capacitor aging
679 watts + 40 watts for the OC on each GFX card = 759 watts
@ 90% load
Can ya get away with less ? Certainly ..... but my experience has been when I skimp on PSU and the power draw at the most extreme OC's you wanna tackle are on the line.....the further you are from that power rating, the less strain on the PSU. The lower the strain, the less noise, the less heat, the less voltage variation and less ripple...... all these help with OC stability. With the HX series on a $2k build the cost difference for the 850 HX series from 750 to 850 is 1.5%