GPU needs PCI express 3.0 My motheboard has PCI express 2.0

You will have absolutely no problem with that. You might have some bottlenecks if you want to SLI 2 780ti, but with only 1 780ti it will work perfectly!

I'm not so sure about your CPU, though. I don't know if it can push that GPU to its maximum...

The main reason to buy the overclocked version would be the better "aftermarket" cooler, but if both have similar coolers, then get the non OC version and overclock it yourself!

Backwards compatible means that, even though it's new, it can still be used on older hardware.
About percentage of power, pay no attention, as only 1 GPU (no matter how good it is) will not tax the PCIe 2.0 x16 completely.
I don't think you should buy a weaker GPU, but you might want to upgrade your CPU in a future (although that would require a whole new platform)

As I edited above, your CPU will probably be fine most of the times. Is it overclocked?

1. Backwards compatible means it works on olde MoBos.
2. Your 850 watt PSu will handle two 780 Ti's ...overclocked....the card draws about 262 watts. At stock a 550 watter is recommended (650 if overclocking) and an 800 (850 if overclocking) for SLI.
3. The assumption is oft made that a factory OC'd card is somehow the same as a a reference card except for the cooler. This is true only in some instances tho the industry is starting to lean this way. Form the 5xx series till today, the difference between the reference card and the 3rd party card could be as follows:

a) Same danged thing with no changes other than factory OC that you could do yaself. This is the only case where the adage holds that "you can do the same thing yourself".
b) Take the above and add a better cooler.... good example... EVGA Superclocked series.
c) Take the above and add a custom PCB and beefed up VRM. Here's a good example where ya can see the referce 4 phase VRM and other manufacturers using 5, 6 and even 7 phases. This is an inherent advantage in overlcockinmg leading to the caoability to handle\ more voltage at less heat .... both essential to getting those highest OC's and maintaining them for the life of the card.....ie. Asus DCII, MSI Twin Frozr, Gigabyte Windforce.
http://www.tomshardware.com/reviews/geforce-gtx-560-ti-roundup-asus-engtx560-graphics-card-overclocking,2858.html
d) Take the above and add hand picked GPUs for the best overclocks.....ie. Asus 670 DCII TOP
e) Take the above and add dual BIOS, voltage limit bypass, etc. .... i.e. MSI Lightning / RVGA Classified.

Now while this was the normal practice for the 5xx thru 7xx series, with the 780 Ti we are seeing some manufacturers go back to reference PCBs. The MSI 780 Ti manages to trounce the competition getting the fastest clocks while running at the quietest sound level by far. How they managed to outclass all the competition with a reference PCB is quite a feat. Why they went from a custom PCB to a reference PCB is and interesting question and one I have not seen explained but I can guess....

-With the 8xx coming, by the time custom water blocks arrived they wouldn't have much time to sell any of them.
-The VRM was robust enough it didn't need any help.
-Given nVidia's voltage limitations (both contractual and physical), they didn't think it was worth the time and effort.

http://www.techpowerup.com/reviews/MSI/GTX_780_Ti_Gaming/29.html

Last week the MSI 780 Ti was $600 on newegg for 2 days but now back up at $710. Gigabyte has a Windforce card at 1020 Mhz (1150 boost) matching the MSI's speed but it's bit loud....however it is $680 on this side of the pond. There's also a 1080 MHz core model ($730 here) from Giga but it has the same boost clock of 1150 so I'm not sure just what advantage you'd see....still looking for reviews.
I see no limitations on ya current build other than you may see a pinch if and when ya go to SLI....the 960 will do fine. 1366 was a very overclockable platform (Son No. 2 has a 950) and is still using it with a 580..... he graduates from college this weekend and ..... once employed, I'm sure that a new system is in the works