What else will the PSU have to power? The CPU doesn't use much.
The i7 975 is wayyyyyyyy more expensive than the i7 920 - you sure you want to spend that much extra for a small increase in performance?
It really would help if you listed the rest of your components. Which video card do you plan on installing? Are you going to use mutiple video cards? Are you a hardcore gamer into serious overclocking? All of that makes a difference.
The general rule of thumb is a high quality 500 to 550 watt power supply with sufficient current on the 12 volt rail(s) can easily handle a system with any single video card made. A high quality 700 to 750 watt power supply with sufficient current (amps) on the 12 volt rail(s) can handle just about any two video cards operating in dual Crossfire or SLI mode. There are a few exceptions at the high end.
Corsair, PC Power & Cooling, and Seasonic are brands with a reputation for high quality power supplies. They consistently earn high marks in technical reviews. They are stable, reliable, and come with longer warranties.
1. Hit Calculate just to make sure all is clear (should see 34 watts)
2. Add your 975 processor
3. Lave TDP at 85% and Hit calculate (should see 134 watts)
What will really determine your PSU size is what goes in next.
1. Add Hi end desktop, hit calculate (146 w)
2. Add 3 sticks of DDR3 (155w)
3. Add a GTX 285 (298W)
4. Add a 2nd 285 in SLI (419W)
5. Add 2 HD's & 1 SSD (420W)
6. Add DVD -RW and a BR Burner (473W)
7. Add one PCI-e card, 5 USB and 1 Firewire (506W)
8. Add fan controller and card reader (523W)
9. Add Antec 1200's eight case fans & twin HS fans (578)
10. Leave system load at 90% and add 25% capacitor aging over time
Total 722 watts of constant power draw. Now multiply by peak power / continuous power ration of say 1.5 and you get 1083 watts. So I will say something between 1000 and 1200 watts for this super system
To see how well we did, lets pop over to slizone and see what nVidia says for twin 285's.
The 3rd drop down is for twin 285's lists several hi end 1000 watt and 1200 watt PSU's. So I won't say that this method is the correct and only way to figure it out; but I will say that it compares well to what the nVidia engineers think is appropriate. Sure you could probably get by w/ an 850 but I'd be more comfortable OC'ing w/ a 1000 watt or better ..... the 850 also being closer to its rating will have to crank up that fan a little more too, likely making it a bit harder on the ears.