First of all let me start off by saying I water cool my personal systems.
1. No.... there is no longer any real performance improvement to be gained from water cooling either the CPU or nVidia GPUs unless you are into extreme overclocking. As often as not, with modern componentry, you hit the voltage wall or manufacturer imposed limits before you hit the temperature wall. Today's 1080s start to throttle at 82C ...here's the temp experienced by the MSI 1080 Gaming at max OC,
If the card hits 82C it's going to throttle, if it doesn't, it doesn't. So.... whether your GPU is at 81C, 74C or 40C, the card will perform exactly the same. What water cooling will do for you is reduce noise.
2. Using a 120mm radiator on a CPU is a waste of effort, a $30 air cooler will do just as well. Putting a CLC with a 240mm on your CPU might seem like a good idea ... It's NOT.
Start at the 17:12 mark @
https://www.youtube.com/watch?v=TivNOgQqW-M
The $100 Corsair H100i manages to keep the CPU at 73C at 4.4GHz ... but the $80 Noctua Air Cooler manages to keep it 2 degrees cooler ... and to lose by 2C, the H100i has to be 12 times as loud.
3. Yes, they split up the work load but they do more work .... 1 card may max out at 74C doing 60 fps but 2 cards will max out at 72C doing 90 fps. Except for "real life". Because the top card is using exhaust air from the bottom card as intake air, the top card may run as much as 10C hotter.
4. I haven't seen any top tier card with just one fan... it's usually 2 or 3. But don't focus on the minutia... size also matters ... sound also matters, so there is no universal answer.
Sizing radiators is strictly a measure of heat produced. An overclocked 6800k will produce about 125 watts.... each 1080 will produce about 200 watts outta the box, add 20% (240 watts) for the better cards after you raise the power limit to 120%
So .... 2 x 240 watts for the cards + 125 for the CPU 20 watts for the pump = 625 watts
Trough experimentation, we've learned that he rads need to handle about 60% of that load as a) there's never a condition where everything is at 100% b) it's an average not peak load that matters for cooling and c) your rad shrouds, heat sinks, backplates, and all component surfaces are radiating heat.
so 60% of 625 = 375 watts.
The typical design goal for a water cooled system is 10C delta T. Using the data sheets you can download here, we can see how each rad does in this respect
http://www.overclock.net/t/1457426/radiator-size-estimator
If you don't like noise, I'd suggest using 1200 rpm fans which will be dead silent below 850 rpm or so. Looking at the Alphacool XT45 (45mm thick), we see that each 140mm of rad provides about 83 watts of cooling (101 in push / pull. With a load of 375 watts, that works out to about 4.5 x 140mm.
Using say a 420mm rad on top and 280 on the bottom would give you 5 x 83 watts of cooling (415 watts) capability and I'd expect a Delta T of about 8.3C.
Other combinations...
4 x 140mm ... Delta T of 11.3
3 x 140mm ... Delta T of 15.1
Using 120mm fans will reduce your cooling by about 25% (61 watts per 120mm)
You can use higher speed fans but it defeats the purpose of water cooling... at 1800 rpm for example that XT45 will generate 117 watts of cooling per 140mm (41% increase) ... but again, you will hear the fans at anything above 850 or so rpm.
My typical water cooling design for a SLI system uses a 420mm (45mm thick) rad on top and 280mm (60mm thick) rad on bottom. If you find a custom loop a bit scary, consider using a Swiftech H20 X2 w/ twin MSI GTX 1080 Seahawk EK X .
I use:
EK Supremacy CPU Block -
https://www.ekwb.com/shop/water-blocks/cpu-blocks
Swiftech Dual DDC MCP35X2 Pump -
http://www.swiftech.com/MCP35x2PUMP.aspx
EK Res 3 Resrvoirs -
https://www.ekwb.com/shop/reservoirs/reservoirs/ek-res-x3-series
EK Full Cover Watewrblocks -
https://www.ekwb.com/shop/water-blocks/vga-blocks/full-cover-for-nvidia-geforce
The Swiftech is a pre-assembled set of custom water loop components which can be expanded to include additional blocks and rads very easily. Unlike CLCs, no cheap componentry, no aluminum rads, no mixed metals. The cards uses an EK full cover water block and the MSI cards are free from the issues that plague the EVGA SC and FTW lines tho to be fair, water cooling kinda solves those issues.
http://www.swiftech.com/h240x2.aspx
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127952
You'll need to buy some extra fittings, tubing and coolant to connect the blocks.
As for the build ...
1. I was not all that impressed with the PG348Q. G-Sync is nice and all that but with no Motion Blur Reduction (ULMB) I really can't see paying that much for a monitor. Until Display Port 1.4 monitors arrive, I have not been recommending IPS screens above 1440p. There was a bug w/ the overdrive control mentioned in the TFTcentral review... i dunno if it was fixed yet
2. With the 1080s expected to drop soon, i think I'd be waiting for them either to get them or take advantage of the price of the 1080s once the Ti hits.
3. As for which 1080 to buy, nVidia and Boost 3 have made choosing somewhat hard as they all perform relatively the same. Techpowerup gave them all a 9.8 rating so it's hard to argue for one over the other....except for the EVGA models which are having the VRM issues.