Replies:
1) You will have a bottleneck with your CPU as you've already confirmed yourself, so you really need to build a new system with an i5-4670K based on the amount you seem to be going to spend. Some would argue for an i7-4770K. I wouldn't with a single GTX780Ti but would with two of them.
The i7-4770K would be as high as 30% faster for converting video compared to the i5-4670K if the CPU is the sole bottleneck.
2) PhysX:
If the 2nd card isn't being used you should use it as a dedicated PhysX card. As I said, it needs to be (roughly) at least 75% the speed of the main card. If it's identical then there should be no bottleneck and you'll get higher frame rates than using only the main card for everything.
You can experiment with this yourself though I don't know if you can have it auto-applied per game? I have no option to do that but then I only have a single card.
3) Video Editing:
It's my understanding that none of the programs you mention take advantage of Graphics Cards beyond a certain point (i.e. GT650 is similar to GTX780). You can Google that but I had a link at some point.
The largest bottlenecks would be the CPU or System RAM. Which one it is depends on the program used, how much editing you're doing, and how large the video file is though I'd use 16GB as the minimum. 8GB is optimal if only gaming.
4) Games:
You can look up benchmarks easily yourself. They assume a good Intel CPU usually like the i5-4670K. You can find 5760x1080 benchmarks. Also, just HALVE the framerate for 3D.
GTAIV is an exception. It's coded strangely and runs like crap on even good systems. It gets locked to 44FPS and 52FPS depending on my settings, plus you can't apply anti-aliasing. GTAV however is very well done for PC (read about the development) so will be great when it comes.
5) SLI:
Don't even consider the GTX760 for triple monitor, and make sure you get at least 3GB per graphics card such as the GTX780 has. If you really want triple-monitor and/or 3D you need at least a 2xGTX780 setup with a good Intel CPU.
6) GTX480 for third monitor?
I'm confused, didn't you want triple-monitor gaming? You won't get that if you have one of the monitors on a GTX460 and the others on a different card. Here's how it works:
http://www.guru3d.com/articles_pages/geforce_gtx_780_sl...
I'm pretty sure you can use a 1x or 2x setup with three monitors as well but you can look into that. I really wouldn't go above 2x as there are a few issues with 3x.
Anyway, there's no good reason to have a GTX460 sitting in any system just to run a 3rd monitor. Even if you needed another card which is unlikely, you'd be better off with an inexpensive HD6450 512MB or similar just to do basic tasks because otherwise you're dumping more heat and noise into the PC than you really need to.
I'm also confused by the 3rd monitor comment in that are you thinking about gaming on only TWO monitors? If so, that's a bad idea as the gap between two monitors is right in the middle which sucks.
The GTX780 supports four monitors I believe assuming you have monitors that support all the outputs. If you don't have DisplayPort it would likely be 1xDVI, 1xDVI, 1xHDMI (HDMI->DVI adapter if needed).