Many GTX750Ti's have a 6-pin power connector,

Thanks for the tip, I've checked only the pictures on the ASUS, Gigabyte, and MSI website, and they doesn't show the top side of the card.
There is an example of the opposite: the MSI GTX 750Ti 2GB Gaming OC doesn't have a PCIe power connector...

A long time ago I had four GT240's in the one system (69W TDP each). The GPU's didn't have their own Power connector. The motherboard had an additional power connector (might have been Molex, but some have a 6-pin PCIE connector and or an extra 8-pin power connector),

Look just above the top PCIE slot, and just above the CPU.

The second extra PCIe power connector is actually on the left side of the bottom PCIe slot, the connectors above the CPU is for the CPU :).

Best for a single slot would probably be a dual GPU Titan Z, though I loath the damn thing and NVIDIA for having the audacity to try and sell it due to its exceedingly ludicrous price point, having lower performance than two Titan Blacks but costing a half again more (1000 USD) than their combined price. It's better just to build another computer and get another card for it at that point.

My computer setup is optimized for 2-way SLIed cards with each card having 16 PCIe gen. 3 lanes.

My two EVGA Titan Black Superclocked Cards together (each running one work unit at a time) at stock boost clocks and power targets while being on average about 75% loaded each are so far (I started crunching for GPUGrid this last weekend) averaging about 1 million credits a day exclusively for GPUGrid. (Amazingly, this is about 20 times the amount of credit I was getting per day with the other projects I crunch for before dedicating my GPUs to GPUGrid.)

1 GPUGrid tast per card seems well enough for me, though I could push it to run more. I want to keep my cards cooler and not have them stressed as much. This way I can also game, etc. at the same time too without changing any BOINC settings around.

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My BOINC Cruncher, Minecraft Multiserver, Mobile Device Mainframe, and Home Entertainment System/Workstation: http://www.overclock.net/lists/display/view/id/4678036#

Just to help clarify: The Titan Z is essentially the same exact thing as two underclocked Titan Blacks on one card, though as mentioned, for half again more the price.

Anyway... yes, of the Titan style of NVIDIA cards out there (in all but name, memory capacity, and double precision performance - which are all essentially irrelevant as far as GPUGrid is concerned), the 780 Ti cards are among the most powerful and cost effective.

To put this in perspective: Someone could potentially get four 780 Ti cards and have more than double the performance of a Titan Z while paying about 200 USD less overall.

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My BOINC Cruncher, Minecraft Multiserver, Mobile Device Mainframe, and Home Entertainment System/Workstation: http://www.overclock.net/lists/display/view/id/4678036#