How many GPUs can I use?

[puppydust]

Hi everyone

Sorry for the noob nature of this post - I’m quite new to the world of Octane and GPUs (and even PCs!) and so just looking for a little bit of advice on building a rig.

I’ve running a PC with i7 3930K and P9X79 motherboard with a basic GPU (GTX 550Ti).
It seems like this motherboard (https://www.asus.com/Motherboards/P9X79/specifications/) only has two PCI-express 16x sockets on it, so does this mean I could only use two GPUs? Or can I use the x8 mode one too?
What confuses me slightly are posts like this where the user (with same motherboard/CPU) talks about being able to fit up to 9GPU's... how?! viewtopic.php?p=276553#p276553

(The system is also overclocked and I'm running a 650W Corsair PSU with H100 water cooler)

I am essentially looking at buying some better GPUs for my system, so currently looking at kitting up with 980s or perhaps a greater number of 780s depending on how many I can use. Searching on eBay, there are lots of different variants to each kind of card. I’m totally new to this, is there any difference performance wise between them? Is there anything I need to watch out for, and can I mix and match? Do I need to consider water cooling?

Thanks in advance for any advice.

[preciousillusion]

You can use any PCIe, x1 - x16. To use 9 GPUs you'll have to use PCIe splitters.

If you're considering 780s you might want to take a look at 970s instead, performance is similar but they draw less powers and got 1 GB more RAM.

You're going to need a more powerful PSU for anything more than a single 780/980/high-end GPU, especially if you're overclocking.
One thing to be aware of when using more than 2 GPUs on a motherboard without support for additional PCIe slot power is that it's going to pull a lot of current through the 24-pin ATX socket, an additional 6 Amps per card at full load.
And, as I learned the hard way, depending on the rating of motherboard components/connectors, or oxidation, there's a risk that something on your motherboard blows/burns/catches fire.

[puppydust]

Hi preciousillusion thanks so much for your advice here.

I wasn't aware of PCIe splitters.

The thought of modifying my system myself without expertise gives me nightmares somewhat of the whole thing exploding and setting fire to my office while I leave it overnight
What's the best way to handle the increased load that the GPUs present, as a result of the current increase you mention.
Or in short - What's the best way for a newbie like myself to go about adding the extra GPUs? Would it be best to get my system sent back to the company that build it to get it tuned the correct way? Or if I added a beefier PSU myself would that do?

Just FYI, the system is overclocked but I hadn't intended to overclock the GPUs.

[puppydust]

Also, is there any difference between the different 3rd party models for the cards?

For instance I have seen on eBay versions of the 980 by Gigabyte or by MSI or by eVGA. Is it a good idea to mix and match?

[Dizern]

Hi there!

I plan to invest money in 4-5 cards external farm for rendering/mining. As I understand the difference in x1 and x16 pcie is scene-loading speed, how much of a difference does that deal?
I'm just thinking about motherboard choice...do I need 5 pcie slots or it;s not a big difference with x1

Also, if I need to go with x16, i need to get x16 to x16 risers, right?

[preciousillusion]

Hi there!

I plan to invest money in 4-5 cards external farm for rendering/mining. As I understand the difference in x1 and x16 pcie is scene-loading speed, how much of a difference does that deal?
I'm just thinking about motherboard choice...do I need 5 pcie slots or it;s not a big difference with x1

Also, if I need to go with x16, i need to get x16 to x16 risers, right?

If it's PCIe 3 x1, you've got 1 GB/s bandwidth so there's not going to be any noticable difference.
You can use x1 risers in x8/x16 slots.
You have to check motherboard specifications carefully as most of them doesn't support all PCIe slots being occupied at the same time.