I have checked the other threads and am un able to find a real answer to this.
I was considering adding a second video card in SLI mode.
However, am uncertain if it would add any real benefit.
My current system contains:
I7 990X-CPU
16GB RAM
Gigabyte Assassin Motherboard
PNY GTX580 OC
Would I reap any REAL Gain by adding a second card in SLI mode ( as far as veiwport or rendering ) ?
Thanks for any information in advance
Only for rendering i believe.
No, at least not its native render engine. SLI and Crossfire are for gamers, they split the workload allowing for higher frame rates. Rendering in Blender is done by the CPU (if you want a quality render) not the graphic card. There are a few rendering engines that use CUDA, a proprietary API by Nvidea that does use the GPU. I believe there is a plug-in or script that can let Blender port a scene to one of these engines.
Well then, I guess its a No-Brainer.
It is just not worth the $500 bucks.
I don’t do games so it will do me no good.
Thanks for the info.
You don’t even have to have “SLI”. Any extra, reasonably new video card (doesn’t have to be the same card) will help you with rendering in octane, or SLG, the two main contenders for rendering using your video card that I’ve seen. Both are fairly easy to get your blender models into. Cycles is also very good but doesn’t support multiple cards last I heard. I believe it’s also possible to use an amd card with an nvidia if you feel like it. But from what I’ve seen I don’t think their performance/price ratio for rendering is quite up nvidia levels yet. recent drivers have improved quite a bit tho. Of course buying a different card would mean you’ll loose out on your sli for gaming 
I’m not really sure, but I don’t think that sli would help much in the viewport department for blender, but please, if someone has some firsthand experience please chip in, I’m also interested!
Don’t mix nVidia and AMD cards unless you’re ABSOLUTELY SURE your motherboard can handle it. Otherwise you’ll end up in driver hell, and possibly two useless video cards.
SLI isn’t what you would benefit from with two cards, as it’s almost solely for rasterization engines. However, you would benefit from SLI for any physx or CUDA calculations, such as with Octane Render or upcoming Cycles updates able to use multiple cards. Some motherboards can only use one 16x PCI-e bus at once though, and will default to 2 x 8xPCI-e when you throw in a second card, which could present a bottleneck with code that utilizes 100% of both cards for anything. Just things to keep in mind.
I had actually purchased this GTX 580 OC card because
it is about as good as you can get without purchasing
a Quadro card.
It is capable of but not limited to
NVIDIA CUDA
DirectX 11
OpenCL
DirectCompute 5.0
OpenGL 4.1
So, as far as a single card, it possesses one hell of a skill set.
BUT, for $500 I am just not sure if it is reasonable to get
another one.
I’m pretty much in the same boat, bought a 570 OC, which I’ve easily managed to get to 880mhz and can outstrip a stock 580, but perhaps not yours 
If you do get into gpu rendering though, a 560 ti at a later date might be a good option (if you’ve got room and money; I don’t) to add some clout when they get cheaper.
GPGPU:
If you´re in GPGPU raytracing you get a truckload of GTX285 and OC them. One of them is almost as fast as a Stock GTX470 and costs 60 bucks on ebay, or get a GTX470 for 100 bucks and OC it to 750-800. You get 5 GTX470 or 8 GTX285 for what one 580 costs…
If you use it for octane, you got to wait for the beta 2.5, as the current version has troubles with multiple cards.
Also the gtx285 has only CC1.3, while from the GTX400 series on they have CC2.0 but I think Octane is downward compatible.
SLI:
If not especially coded, SLI runs with alternate frame rendering under DX. For OpenGL it does nothing, unless the application is SLI enabled and has the code for it. So you´ll most likely have nothing at all from SLI+Blender.
AMD+Nvidia:
No problem at all. If your board has more than 1 PCIEx16 specified slot you can simply combine them in Windows Vista or newer. Already did it on several occasions.
GeForce vs. Quadro:
Not going there for the 10.000th time. The GF400/500 series has crippled OpenGL support and runs extremely slow compared to a GTX285 or all AMD cards. If you´re unaware of that, use the forum search.
Thanks for the information.
It has been Very helpful.
Based on what I have read here, I have decided to not get that second card.
You could easily think that this might be a Cut & Dry situation. Imagine my
suprise when I have discovered it is NOT.