If you have enough space you can always buy another power supply and turn it on via the sophisticated paperclip technique :
http://aphnetworks.com/lounge/turn_on_p ... clip_trick
Or get those specialised switches (which do the same thing)
I did mine with the paperclip, and I got a 950W corsair, along with the 750w corsair PSU, two GPU card on 750W unit and two on 950W unit (on asus workstation motherboard with 7 PCIE-s).. heats up like hell, placed a few large fans all around the cards and the temps don't go above 80-90C, it's loud but it works
And check out this app http://www.ozone3d.net/gpu_caps_viewer/ , has everything to cheack out the GPU, GPU load, TDP power on each card so you can calculate the total wattage I guess..
Power supply and 2 cards
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Please add your OS and Hardware Configuration in your signature, it makes it easier for us to help you analyze problems. Example: Win 7 64 | Geforce GTX680 | i7 3770 | 16GB
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Thanks for those links! Those are cool. I would definitely have to do it that way if I really expected to put a third card in. But come to think of it...my only other slot is a PCIe-x8, so either that would cut my bandwidth in half for the second PCIe-x16 slot, or it wouldn't work at all. I'll definitely have to upgrade to a bigger motherboard sometime in the future.
Win7 | Geforce TitanX w/ 12Gb | Geforce GTX-560 w/ 2Gb | 6-Core 3.5GHz | 32Gb | Cinema4D w RipTide Importer and OctaneExporter Plugs.
My specs: look at my signature.
My observations: (all measured)
Power consumption:
1. At idle approx. 150W
2. Rendering with one GTX 580 (watercooled) - add approx 200W.
3. Rendering with two GTX 580 (watercooled) - add another 200W.
Total power consumption never exceeds 650W. I have 1350W PSU - for further 2xGTX580
This should not exceed approx 1100W.
All work at 30C - idle, 45-60C while rendering. Silent.

My observations: (all measured)
Power consumption:
1. At idle approx. 150W
2. Rendering with one GTX 580 (watercooled) - add approx 200W.
3. Rendering with two GTX 580 (watercooled) - add another 200W.
Total power consumption never exceeds 650W. I have 1350W PSU - for further 2xGTX580

All work at 30C - idle, 45-60C while rendering. Silent.

3090, Titan, Quadro, Xeon Scalable Supermicro, 768GB RAM; Sketchup Pro, Classical Architecture.
Custom alloy powder coated laser cut cases, Autodesk metal-sheet 3D modelling.
build-log http://render.otoy.com/forum/viewtopic.php?f=9&t=42540
Custom alloy powder coated laser cut cases, Autodesk metal-sheet 3D modelling.
build-log http://render.otoy.com/forum/viewtopic.php?f=9&t=42540
That's some major cooling going on there. I could use that stack of fans for ceiling fans! 
Really nice setup.
A question comes to mind. What percentage of max wattage should a power supply never exceed during normal operation (not including spikes)?

Really nice setup.
A question comes to mind. What percentage of max wattage should a power supply never exceed during normal operation (not including spikes)?
Win7 | Geforce TitanX w/ 12Gb | Geforce GTX-560 w/ 2Gb | 6-Core 3.5GHz | 32Gb | Cinema4D w RipTide Importer and OctaneExporter Plugs.