Sli is very straight forward, no need to do anything special apart from hook both cards up, install the bridge and enable SLI in nvidia control panel.
While you can do full clean driver installs, but i only tend to do that if I encounter a serious issue otherwise i just do the express install (custom, i dont need 3d vision driver)
in regards to over clocking while in SLI if this interests you, I tend to disable SLI and test both cards individually to see their OC limits while in single card, then re-add both cards and enable SLI to test them together.
It's way more simple than it sounds and in regards to heat I personally find after-market cooler designs better and quieter while in single card or SLI considering most of us are on ATX size you will be fine, though the slot between the cards matters (Smaller gap = more heat) generally though using MSI after burner and setting a decent fan profile up (Google if not doing this!) will sort your problems long as you dont go too far over 70c as I believe 780s like their previous brother GPUS throttle back in small Mhz increments to bring it back to 70c or under although in real world testing you wont notice a thing.
As for power your 850w PSU is MORE than capable, to put it into perspective with Everything in my rig (see sig) running balls to the walls full, both GPUS overclocked to their limit (1202 core/7008 memory) i only pull 500-550w from the wall and most PSU's are under-rated IE: your 850w is probably something more like 920-950w to compensate for capacitor aging.
one thing I will say is SLI can be a great thing and a not so great thing it entirely depends on the game you wish to play, some games utilize SLI amazingly (crysis 3 for example running maxed out @ 1080p , smooth as butter and over 70FPS) and some take SLI and turn it into a p.o.s thankfully it's very easy to disable SLI for that game (Nvidia control panel>game profile>single gpu only) and even it's possible to use a program called Nvidia inspector to tweak the SLI profile and potentially make it run better/utilize the cards more [All information can be had via a quick search].
It can be a bit of a faff at times, you will wonder why a game doesn't run that great with both GPUS and find on google that the coders where lazy but that's happening less and less now days and generally a single card will serve you fine in those games anyway.
Concerning old games:
Most will NOT need your SLI to achieve desired quality/fps again just disable via the driver and you should do this to save power and heat.
After over a year with SLI cards I can honestly say its fun to tinker with, it's rewarding and on the games that use SLI and use it well .......POWER!! your face will light up with joy! that being said I have spent countless hours on google searches/OCUK forums trying to find solutions/improvements and tweaks but this is my hobby so I'm more than happy to do that and the benefit has been huge.
So don't be afraid of your purchase but it never helps to do a little research before ordering
Tip:
Clean cables help airflow
Give PC a de-dust before installing, including your original first card and power supply.
Take it easy and go slow in the excitement
Make sure you have decent airflow
read up on Nvidia inspector it's a very useful program with some nifty advanced features
including the ability to force better anti aliasing on games.
same with MSI after-burner if you are not already using it or EVGA precision, even if your not overclocking the ability to set a better fan speed curve is highly desirable.
Enjoy the good times and dont let anything I have said put you off, It's really a lot more simple than it sounds.
. The P8p67 pro Realtek audio is not 'HD' at all