Well, since you asked. My top host currently (soon to be passed by the other) has the following MB

http://www.newegg.com/Product/Product.aspx?Item=N82E16813128514

Spec say PCI Express 2.0 x16 4 (x16, x16, x4, x4)
so here is how I have it set up. There is a PCI-e X1 just above the first PCI-E x16 that I use a PCI-e Riser 1 to 16 with Molex connector to a capacitor. Here's an image of it



so in sequence (only the last 3 are actually on the MB) PSU 750 W

PCI-e X1 GTX 750 TI SC (this one is rigged to the side of the case) Molex to riser
PCI-e X16 GTX 650 TI Boost with a 6 pin connector from PSU
PCI-e X4 GTX 650 TI Boost with a 6 pin connector from PSU
PCI-e X4 GTX 650 TI (no boost) with a 6 pin connector from PSU

The other computer is my new build with a better MB and Case.

http://www.newegg.com/Product/Product.aspx?Item=N82E16813128508

All GPUs are on the MB. PSU 1300W

PCI-e X16 GTX 750 TI SC with ACX with a 6 pin connector from PSU
PCI-e X8 GTX 650 TI Boost with a 6 pin connector from PSU
PCI-e X16 GTX 650 TI Boost with a 6 pin connector from PSU
PCI-e X8 GTX 750 Ti SC (no connector)

Hope this helps.

Happy Crunching..

Zalster

... Their owners would have to be asked to confirm.

My computer 7265951 has 4 GTX750Ti WITH the 6-pin PCIe power connections, and I use them. I hate smoking expensive equipment! :-)

As an aside, when running full tilt, the power use indicated by GPU-Z is about 65-76% which is 40-45 watts averaged over 1 second.

[EDIT] My MB is Gigabyte GA-Z87X-OC.

---

I still can't determine how a manufacturer arrives at a maximum number. Let's consider the Intel board I have. The manual says the max combined PCIe wattage is 300 watts for 4 full sized slots, 2 at x16, 2 at x4. Does that include the Two 75 watt Aux cables that are attached to the motherboard? Doesn't say, but they sold that model with the option of 4 2600XTs. Obviously you can exceed the 300. Some people installed 2 5870s rated at 188 watts apiece...376 watts. Some of those people fried their 75 watt cables. But, what about Two 150 watt cards with Two 75 watt cards? The cables aren't a problem but that's 450 watts. It has a 980 watt power supply and was designed for two 4 core processors and 64GB of Ram. If you have just One 4 core processor and 6GB ram does that mean you can use more for the PCIe slots? I can't seem to find any info on it. I'm currently running at 333 watts and all is well. I'd like to go to 353, but, I'm scared :-)

Meanwhile, if you look at the top computer list you will find a few nVidia Hosts with 3 and 4 GTX 750Ti cards, http://setiathome.berkeley.edu/top_hosts.php?sort_by=expavg_credit&offset=40
So, it's working for some people.

Only 1 of those hosts that displays "[4] NVIDIA GeForce GTX 750 Ti" actually has 4 of them. The two machines that belong to Zalster have 2 & 3 650 TI Boost in each machine. Given those are 130w cards I would guess the 650's are connected with PCIe power cables, but you would have to ask them to be sure. Then again about half the 750 Ti cards do have power connectors on them. Maybe all of those machines have power going to the cards. Their owners would have to be asked to confirm.

The HD 2600 XT is a 50w card. So I don't see any problem putting 4 of those in that MB you mentioned. The fact they give you a power spec for PCIe cards is nice. I haven't seen that information provided with any of my boards.
The maximum power they defined for your board is probably based on the traces built into the board. If they used 20% overhead in theory up to 360w through the MB. After that things might start to get sketchy. The only way for sure is to do a smoke test.

After long consideration, and personal experience, I'm going to say Apple is talking about their PCIe slots only. They don't mention the Aux connectors and no matter how much new math others try to invoke, the numbers only work one way. The easiest link is here;
*This model has two PCIe x4 expansion slots and two PCIe 2.0 x16 expansion slots (in the default configuration one PCIe 2.0 x16 slot is occupied by the graphics card). Apple also reports that "all slots provide mechanical support for 16-lane cards [and supports] 300W combined maximum for all PCI Express slots."
300 watts is exactly what it should be according to the PCI Express standard. Only way to be sure is call them....I guess.
Those people that ran into the Aux cable problems just placed another power supply next to their machine and went on about their business, if melted wires were the only problem...

BTW, if anyone runs out of Aux cables the solution they gave in this thread was this unit, EP-450CD 450W ATX12V / EPS12V Modular Juice Box
That's pretty cheap, has anyone tried one of those?
That thread was kinda funny. The guy went on and on about using the splitter cable then in the end agreed you might need to buy the power supply. Note no one mentioned a 300 watt 'combined' limit. Seems the only reference anyone can find mentioning the Aux cables in that limit is in an old 2007 Training book. There was a big change in the 2008 model's PCI system. I spent way too much time on that...

Just thought I'd follow up on the PCIe x1 Mod mentioned above. I moved my 75 watt Gigabyte 7750 to the HP xw4600 board and tried it in the Open Ended x1 slot. The Gigabyte 7750 doesn't have an Aux cable, it was inserted directly into the motherboard slot without using any additional items. Works perfectly fine with my XFX 6850 in Ubuntu 12.04. I ran one complete task, Task 3561484841.

  Max compute units:				 8
  Max work group size:				 256
  Max clock frequency:				 880Mhz
  Max memory allocation:			 596115456
  Cache type:					 Read/Write
  Cache line size:				 64
  Cache size:					 16384
  Global memory size:				 970981376
  Constant buffer size:				 65536
  Max number of constant args:			 8
  Local memory type:				 Scratchpad
  Local memory size:				 32768
  Queue properties:				 
    Out-of-Order:				 No
  Name:						 Capeverde
  Vendor:					 Advanced Micro Devices, Inc.
  Driver version:				 1348.5 (VM)
  Version:					 OpenCL 1.2 AMD-APP (1348.5)

Unfortunately, the Gigabyte 7750 has the exact same trouble as my MSI 7750 in Vista. Everything goes fine right up to when the AMD driver loads, then a dialogue pops up saying the "Desktop Window Manager has stopped working" followed shortly by the CCC crashing. That's it, it's over at that point. I once tried to run BOINC after that but it just Errors out all the GPU tasks. It does work well enough to catch a screen shot though.

Fixed it. Seems you have to have SP1 to have Vista work with an AMD 7750. Works now. But, Ubuntu works better :-)