So, I am thinking of building a machine to do many things at home, things like recording my cable TV shows via Windows Media Center as well as recording any security cameras onto my Raid5'd drives, using professional Adobe photo/video/audio editing tools, Ableton/NativeInstruments type music production, software development including running VMs for testing, very minimal --if any-- gaming, etc. It won't be cheap. Of course, when I am not using it, it'll do BOINC. I am trying to balance CPU cores and GPUs to maximize the output for BOINC, not only for GPUGRID but for the 40 or so projects I am crunching for. I thought I'd first ask here to get a sense of what people think, since GPUGRID is the most hardware-centric project that I know of, with experts to match, and probably lots of people who would love to build such a system if the lottery knocked on their door... So, let me hear your opinions.

I am looking at 4 configs:

1) Liquid cooled Intel Extreme Core i7 5960X 8-Core (16 threads) 3.0GHz (3.5GHz TurboBoost), 64GB 2400Mhz memory (64/16=4GB/thread), Quad Liquid Cooled NVIDIA GTX Titan X (4*3072=12288 cores, 4*12G=48G, 1075 Mhz, SINGLE precision)

2) Liquid cooled Intel Extreme Core i7 5960X 8-Core (16 threads) 3.0GHz (3.5GHz TurboBoost), 64GB 2400Mhz memory (64/16=4GB/thread), Dual Air Cooled NVIDIA GTX Titan Z (2*5760=11520 cores, 2*12G=24G, 876Mhz, DOUBLE precision)

3) Liquid cooled Dual Intel XEON E5-2699 v3 18-Core (2*36=72 threads) 2.3GHz (3.6GHz TurboBoost), 128GB ECC Registered 2133MHz (128/72=1.78GB/thread), Dual Air Cooled NVIDIA GTX Titan X (2*3072=6144 cores, 2*12G=24G, 1075 Mhz, SINGLE precision) [not Quad like in #1 above since dual CPU & memory banks reduce number of PCI slots on the motherboard]

4) Liquid cooled Dual Intel XEON E5-2699 v3 18-Core (2*36=72 threads) 2.3GHz (3.6GHz TurboBoost), 128GB ECC Registered 2133MHz (128/72=1.78GB/thread), Dual Air Cooled NVIDIA GTX Titan Z (2*5760=11520 cores, 2*12G=24G, 876Mhz, DOUBLE precision)

I am leaning towards #4 given the insane number of CPU threads with minimum loss of memory per thread while getting a very slightly higher max CPU clock, while taking a slight drop in GPU cores & clock speed (compared to Quad Titan X), but keeping Double precision for those BOINC apps that need it now and new projects that might need it in the future.

I am purposely not selecting Quadro cards since I don't want to block any future gaming just because I don't play any games right now. All this graphics power might change my mind... :-)

I wish there was a Titan X dual GPU card with double precision (i.e. the Titan X version of Titan Z), but it seems that will be based on Pascal in mid to late 2016...

Yes, I am aware these are "out there" configs. But being able to choose one over another based on good reasoning would give me a nice direction even if I decided to build a lesser machine.

Thanks for any insight
Tuna
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Your question is not easy to answer, because the answer depends on what purpose should we prioritize.
If I give priority for GPUGrid, then the first configuration is the best choice, it'll be the most energy efficient & futureproof.
If I give priority for tasks / projects needing DP, then the second and the fourth is the best choice (the latter is better).
If I give priority for CPU tasks then the third one is the best choice, while this still could do a great job here at GPUGrid.
So if I would have this amount of money to spend on hardware, probably I would build two machines.
PS:
1, I think there's no point to put security cam recordings on raid volumes.
2, If you use all of the CPU cores, don't expect that those cores will run at maximum turbo boost.
3, The Titan-Z blows half of its hot air into the case, so choose the case and its cooling to blow this hot air out of the case through its front.

So, if you had the money, you'd build #1 & #4?

The raid volume would just be the data storage part of the case. Cases I look at have 5 bays, using 8T Hitachi drives, Raid5, giving one contiguous safe & secure (5-1)*8T=32T usable space. Everything would go there from docs to recorded shows to browser temp files; I wasn't trying to assure safety of any security footage, specifically. Plus probably a 2xSSD boot, Raid 1 or 0.

Good tip about the Titan-Z air blowing characteristics.

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So, if you had the money, you'd build #1 & #4?

The raid volume would just be the data storage part of the case. Cases I look at have 5 bays, using 8T Hitachi drives, Raid5, giving one contiguous safe & secure (5-1)*8T=32T usable space. Everything would go there from docs to recorded shows to browser temp files; I wasn't trying to assure safety of any security footage, specifically.

Plus probably a 2xSSD boot, Raid 1 or 0.

I'd go with the 72 threads to run Rosetta. But that's just me.
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Personally, I would definitely go with the first setup. Unless you have a specific need for double precision, 64bit floating point calculations, there's no reason to worry about them. Only rare and specific applications need to make use of them. My EVGA Titan Black Superperclocked cards have relatively high double precision performance, but yet I don't use it.

Also, with Titan Xs, you'll have much higher performance and twice as much memory per GPU to work with than the Titan Zs to load up more tasks on, even while SLIing them. (I SLI my Titan Blacks for game performance while also crunching 4 GPUGrid long run tasks on them in the background.)

These reasons aside, I kind of hate Titan Zs on principle alone as when they were released they were a half again more expensive and are worse performing in every possible way than two Titan Blacks for basically no reason other than for Nvidia to see what it could get away with, yet they promoted it as if it was some great new thing. Titan Zs are essentially two underclocked Titan Blacks on one card that uses 3 slots worth of space instead of 4 with the stock coolers. Unfortunately, I fear that a lot of people with more money than brains fell victim to Nvidia's marketing as I haven't seen any real comparisons out there for these specific cards.

Anyway, I digress...

The first build is definitely what I recommend in general overall, and I have looked into getting it myself. Check out the Asus X99-E WS motherboard. This is the board I would go with making this build. I use their P9X79 WS board and have had great success with it. Very capable, stable, and a lot of nice overclocking features.
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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#