Running games only at 1080p, there is no need to run out and upgrade. You may see some performance increase at these resolutions, but  should you look into higher resolution 1440 and up monitors and or surround. The 780 will  be a better choice at this point. 3gb will be more than enough for today's games at 1080p.
 
You may want to check out Techpowerups review

1. Depends on your needs.  Will it outperform the 680 Classy? Yes.
2. Will depend on your environment/overclocks/airflow but shouldn't really be getting past mid 70's with adequate ventilation 
3. There may be situations where you could get close to using all that VRAM but for most scenarios you'll do fine. 

I'll do some research on that. My airflow in my computer is very good! my own 680 idles at 25c and never gets above 55c on most games. Thanks for your answers!

Yeah I have a 780TI Classy I will be putting up on the for sale area once I hit 50 posts... Been outstanding, but I am going to a smaller micro ATX setup and it just wont fit!
 
If you do upgrade - Get the back plate - dropped the temps on mine some, and it looks out - standing!
 
 
 
 

Correct.. See my post above for a pic of the backplate on it.. :)
 
 

Yes it is in the LN1 bios .. You need to switch it over to LN2 then you can push it further.. Just note it really wont go much further without water unfortunately.. 

I upgraded because I wanted to. And that card is a true monster, there's literally nothing this card can't do at 1080p. There are many people who say it's a waste of money to upgrade from a 680 to the 780 Ti, let alone to play at 1080p with this card but whatever. I had to get more performance power for the upcoming games so I upgraded and so far I'm a bit over 2 months together with her. And I love that thingy, never regretted getting it although it's damn expensive.
 
My card idles at 25-27°C depending on the temperature outside and inside my room.