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I read there is a paperclip trick to get a power supply to turn on without a motherboard.  I want to connect a power supply to an additional three 7950's connected to two different motherboards which already have power and video cards.  Ran out of connectors from the current two power supplies.  So, if I get a single power supply which will only be connected to three 7950's, how many watts should that power supply be?  

I was thinking about a 750 watt? or maybe 800?

When overclocked I think those cards can pull close to 300 watts each, I would go with a 1000 watt psu.

I don't know if it matters but they are the XFX double D 7950's.

You can run 3x 7950 off a good quality 850w if you undervolt them. If not go with a 1000w.

Thanks for the replies.

Im running 4x 7950 with a 1050W gold psu.

think of undervolting your card a bit, saves a lot (heat and cost).

Be careful here.  Other than the obvious, checking you're shorting the correct pins you need to look at the minimum load specifications for the power supply.  A lot of PSUs specify a minimum load on the other supplies e.g. 5V.  You need to check this out before going any further.  You may find you need to add a few power resistors to generate the minimum loads to ensure proper regulation.

If I plug the 24 pin into an old motherboard that won't be used, but turn on the power supply by turning on the motherboard should that work?

Hard to say.  Again, check the load specs for the PSU you have in mind.  If there's no CPU, memory or fans connected to the mobo your load may not be within spec on 3.3V, 5V and 12V.  Your power supply manufacturer spec sheet should have the answers, choose wisely and you may be fine just with the paperclip though rigging power resistors isn't too hard.  Take a look at:
http://www.playtool.com/pages/psuunderload/mincurrent.html

If anyone can suggest a 1000 watt power supply that doesn't have a minimum, I would appreciate it :)

I be looking for one in the mean time.

http://www.overclock.net/t/15751/info-can-i-use-two-power-supplies-with-one-computer (http://www.overclock.net/t/15751/info-can-i-use-two-power-supplies-with-one-computer)

That's like the paperclip trick I was talking about.  But someone pointed out minimum requirements on different rails.

Yupp...

I have been thinking that it might be better to have a seperate 12V power-supply, and that it might be better and cheaper than a PSU as a secondary electrical source. But seems like most of the stuff I can find are inverters(going the other way.)

A bit off topic, but when you use a second PSU, does it mess with the cards running off the second PSU to have power when the mobo power is off?

You can safely splice the PSUs together so they turn on and off together, or just make a switch:
http://www.overclock.net/faqs/15751-info-can-i-use-two-power.html

Think the 1000 watt version of this would work not being plugged into a motherboard?  Doing the paperclip trick.

http://www.rosewill.com/products/1370/ProductDetail_Overview.htm

Anyone tried this before?  Should be safer than the paperclip, right?

http://www.add2psu.com/

Aye I read that guide about jumping the PSU, but it doesn't go into slaving the 2nd PSU to the first. Would you only have to splice the ps_on onto the primary PSU? Or would you still do both?

Hey I was having same issue with my 5970 ( work for some time and then stop) sets flags to -v -w128 -f1 ..... But anyone know if u can set flags in bitminter?

A different product I use is the Lian Dual PSU adapter cable.  It's cheaper than that doohicky and works well for me.  Best price I found for them was a frozencpu.com

http://www.frozencpu.com/products/5637/cpa-167/Lian_Li_Dual_Power_Supply_Adapter_Cable.html (http://www.frozencpu.com/products/5637/cpa-167/Lian_Li_Dual_Power_Supply_Adapter_Cable.html)

You can run 3x 7950 off a good quality 850w if you undervolt them. If not go with a 1000w.

850 W should be  enough.