You're streaming so obviously Nvidia.
I don't think you fully understand what SLI or crossfire is. For instance if you have 50 fps with a single 780, then you would have with 2x 780's 100 fps, 3x 780's 150 fps and 4x 780's 200 fps but in reality the scaling is so crap after 2way sli that 3 or 4way sli is literally pointless.
I recommended you to stay away from SLI or either CF because your games are barely GPU intensive meaning it won't even max out a single card like a 780 so obviously a 2nd card would be money waste. We still forgot about the issues you have with SLI/CF: loads of noise, microstutter, games running awful with SLI enabled and so on. Just stay away from SLI/CF and stick to 1 card.
Instead of a 2nd 780 just get a 2nd Dell u2713hm (1440p) monitor they suggested and get a 650w psu to save some money because 850w is just brainless overkill for a single gpu rig.
PCPartPicker part list /
Price breakdown by merchant /
Benchmarks
CPU: Intel Core i7-4930K 3.4GHz 6-Core Processor ($568.98 @ Amazon)
CPU Cooler: Noctua NH-U12S 55.0 CFM CPU Cooler ($65.95 @ Amazon)
Motherboard: Asus P9X79 LE ATX LGA2011 Motherboard ($229.98 @ SuperBiiz)
Memory: PNY 16GB (4 x 4GB) DDR3-1866 Memory ($127.99 @ Amazon)
Storage: Samsung 840 EVO 250GB 2.5" Solid State Disk ($159.99 @ B&H)
Storage: Western Digital WD Green 2TB 3.5" 5400RPM Internal Hard Drive ($86.99 @ NCIX US)
Video Card: EVGA GeForce GTX 780 3GB Video Card ($507.27 @ TigerDirect)
Case: Fractal Design Define R4 w/Window (Black Pearl) ATX Mid Tower Case ($99.99 @ Newegg)
Power Supply: EVGA SuperNOVA 650W 80+ Gold Certified Fully-Modular ATX Power Supply ($74.99 @ Microcenter)
Optical Drive: Asus DRW-24B3ST/BLK/G/AS DVD/CD Writer ($25.65 @ NCIX US)
Operating System: Microsoft Windows 8.1 - 64-bit - OEM (64-bit) ($99.98 @ OutletPC)
Monitor: Dell U2713HM 60Hz 27.0" Monitor ($584.99 @ B&H)
Monitor: Dell U2713HM 60Hz 27.0" Monitor ($584.99 @ B&H)
Total: $3217.74
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-12-27 21:12 EST-0500)
I removed the Liquid cooler because there's just no point in it especially with all their issues and terrible noise/performance ratio so I just replaced it with a extremely quiet aircooler the noctua nh-u12s. I switched that Seagate HDD with a WD 2TB Green because it's a quieter drive and a little bit slower but I assume it's just going to be used as a storage drive so it doesn't matter. Switched that Asrock board with an Asus that has the flashback feature so you can update the bios without needing a CPU and you can recover the bios with that feature.
Changed the Corsair 750D for a Fractal design R4 because it's just the most suitable case for single card setups in terms of noise and size. The 750D is more of a watercooling case imo the airflow isn't that great as advertized. If you prefer the 750D over the fractal design, go ahead. I removed on purpose the keyboard because the switches should be a personal choice, the one they recommended was a cherry mx red which is awful for typing. Just google a mechanical keyboard guide and decide which switch is the best for you.
You might be wondering why the monitors are so expensive thats because it's a 1440p monitor with an IPS panel, superior picture quality compared to 1080p. Another advantage of having a 1440p monitor is that you don't have to use so high anti-aliasing settings because of all those pixels you will barely notice jagged edges, like 1x is enough imo