Is it possible to run a third GPU next to a 2-Way SLI?

Question

Is it possible to run a third GPU of the same type alongside two SLI-connected GPUs?

I want to set up a 3-Way SLI system but my GTX660s don't support this.

Does anyone have experience setting up a GPU outside of the case? It would be a tight fit with a third GPU in my case. Is a PCIe extension recommendable?

EDIT: I have checked my motherboard (MSI Z97 G45) which doesn't support 3-Way SLI.

Answer 1

Okay, I got an answer by the nVIDIA FAQ:

Can I add a 3rd graphics card in my 2-way SLI enabled PC to connect more monitors?

Yes, you can add an additional graphics card to your PC to connect two additional monitors. The additional graphics card must have a GPU that is different from your SLI GPUs and must be NVIDIA PhysX capable (GeForce 8 series and higher with at least 256MB of memory). For example, two GeForce GTX 2XX GPUs in SLI and a GeForce 9XXX GT is a supported configuration.

Source: nVIDIA FAQ

Answer 2

As per the quoted NVIDIA FAQ, it wouldn't be possible to run another GTX 660 by itself alongside the 2-Way SLI GTX 660s:

'Yes, you can add an additional graphics card to your PC to connect two additional monitors. The additional graphics card must have a GPU that is different from your SLI GPUs and must be NVIDIA PhysX capable (GeForce 8 series and higher with at least 256MB of memory).'

If you were to run three GTX 660s, it would be best to run them in a 3-Way SLI. However, your edit suggests that your motherboard would not be able to support this.

So it seems that your questions is: Can I run two GTX 660s in a 2-Way SLI as well as an external GTX 660?

From my knowledge of building systems, my answer is: possibly, but it's probably not worth it.

Most external graphic card adapters I have come across are designed to add a secondary monitor to a laptop setup through USB. While some laptops can support an external grpahics card setup, they generally require:

1. A free ExpressCard slot
2. An external power supply
3. An adapter of some sort
4. A graphics card

For a desktop, you would basically have the same requirements. You may be able to power the graphics card by re-routing some of your power supply cables outside the case, however, this is not an ideal solution. You would also need a free 'fast' slot such as an additional PCIe slot. Again, the problem would be your current motherboard as it doesn't have the additional slot for the graphics card.

The best solution here would be to upgrade your setup to support the additional GTX 660 in SLI. By the time you had created a setup that would possibly support the external graphics card, you probably could have just upgraded your setup to accomodate the extra card in significantly less time with less cost.

SLI setups can sometimes be tricky to manage without having to worry about running one of the graphics cards externally! Upgrading your build to properly support all three GTX 660s will be your best long term solution here.