Questions about dual GPUs

I know that Blender can use multiple GPUs at once when rendering so I have been thinking, since I wanted to upgrade anyway maybe I shouldn’t just replace my old card, but instead run a dual GPU configuration. How does this work? I know about SLI, but that is not what I want. Isn’t there a way you can just use a single GPU for the most part, and then you can just switch on another when you need it to render? Say I bought a gtx 780 to use primarily and for gaming and such, could I just use my old card (GTX 660) to help the GTX 780 when rendering? Do I need some additional software or anything?

Secondly I am concerned about my power supply. I have a 700 watt power supply, and a pretty average gaming setup for the rest (i7 3770K processor, 250GB SSD, 2TB harddrive, fan cooling, so on). Do you think it would be able to support a GTX 660 and a GTX 780 at the same time or do I also need to upgrade that?

I know the GTX 780 is not very cost-effective for rendering, and that a GTX 580 would be a better choice, but I am also a gamer so I still want to be able to play the newest games, which is why I have been looking at that card.

Hi, the GTX 500 series is not better for render anymore, since Blender 2.72 the newer cards are better supported.
As you are a gamer too may look for the GTX 980/70, they are a bit slower for render but fast for gaming and need much less power.
You need no additional software, you can switch cards on and off in Blender.
Here is a spreadsheet to compare GPU:

http://www.blenderartists.org/forum/showthread.php?350975-The-new-Cycles-GPU-2-72-Benchmark&p=2751984&viewfull=1#post2751984

There are some power supply finder from different companies:

http://www.corsair.com/en-us/psu-finder

Cheers, mib

Ah very nice! The GTX 970 might what I need then.

One final question and I think I got everything I need. When I install the new graphics card how do I then know which one will do what? I mean if I just plug it in to my other socket in the motherboard, will it just work, or do I need some special connection? As I said I want to use the better card primarily and I don’t know how to control it.

By the way according to this image GTX 970 is faster that the GTX 780/70. So if it is faster for rendering, uses less power, cheaper, and is faster for gaming, then it is just perfect!
http://www.sweclockers.com/image/diagram/8033?k=aff5820dc45689a98ea574d24b51cac2

Hi, on my board I need to connect the card I want to use for display to the first PCIe slot.
On some boards you can setup this in BIOS.
My setup is:
First slot > GTX 760 4 GB connected to display, second slot > GTX 560Ti 448 not connected to a display.
For smaller scenes I use both cards to render, for big scenes the 760 4 GB only.
VRAM is not added!
My tests are different to sweclockers but I know from Octane forum it is possible to get better performance for the new maxwell cards anyway.
I will get a GTX 970 too after next bank robbery. :slight_smile:

Cheers, mib