That PSU makes my heart hurt. You would need no more than 500W for an extremely overclocked, watercooled rig with a single 780 + 4770k. 750W gives you room for two more 780s, although 850W would probably offer better efficiency. For two 780s, 650W is plenty.
The Maximus VI Extreme is shit. It's not worth the money, and is underperforming many $150 Z87 motherboards. Save your money. Get an MSI Z87 Mpower or Mpower Max for overclocking.
You will want a single 360mm rad, or at least 360mm of total rad space, for a CPU and GPU loop. I tend to recommend fat radiators because they offer more cooling options than the slim rads. An Alphacool NexXxos 360 Monsta would be ideal, but the NexXos Full Copper are great too.
Brands? Koolance and Heatkiller are the best block makers, along with Aqualis. For CPU blocks, the Koolance 380i and Heatkiller CU 3.0 are my recommendations. GPU blocks are more subjective, because they are all so close in thermal performance. Look into the Koolance one if you get a CPU block from Koolance, for aesthetics sake, and look into a Heatkiller one if you use a Heatkiller CPU block, and so on. EK is highly over-rated, in my honest opinion. XSPC is great, but I can't stand the new looks they have been going for.
The 840 is a great SSD for reads, but has incredibly low writes. The ADATA SX900 is a better choice for the money, with R/W both over 500MB/s. But, back to watercooling.
The 900D is an okay case, but for watercooling, and build quality, nothing compares to Case Labs, not even LD, as much as I love him. The M8 has support for up to 4x 360mm radiators, which in total, is actually less than the 900D can support in terms of total radiator space, but that does not mean than literally any system you throw at a system with 2 or more 360mm rads will be perfectly fine. The Case Labs M8 was my personal pick for my loop, but I ended up starting to build my own test bench-style case instead. Fantastic case, great guy, and overall, the best cases on the market.
When you design a loop, you have lots of options. You can run multiple pumps in parallel, serial, you can put your waterblocks, if you have multiple GPUs, in series, or parallel, you can run dual loops, you can do all kinds of shit. I recommend just doing a standard lrrp, single pump, parallel everything. The radiators can be linked directly after each other, but it is theoretically better to have your radiators spaced out in the loop order between components, such as pump to res, to CPU, to radiator, to GPU, to radiator, back to pump. It does'n,t really matter that much, in terms of loop order, as long as physically, the pump is below the res in terms of actual distance, but really, aside from that, it couldn't matter less.
Now, your rig as you had it was almost $2500 already. Does the watercooling have to fit inside of the $2500 total, or is it external from that budget? I'm going to assume, for safetys' sake, that it does fit in with the $2500.
If so, then here is what I would build: http://pcpartpicker.com/p/1l4ex
I would probably switch that TIM for CoolLaboratory Liquid Ultra, but they are all pretty close.
A single 360mm radiator, without fans, mind you, is enough for a non-overclocked setup with a single GPU. I could not, despite my best effort, fit 3 good fans in there. However, there is a way around that. If you can spend a little bit more, get 3x Scythe GT 1450 RPM 120mm fans, but if not, get 3x Yate Loons. The are about $5 each, and are fantastic for the price. However, the quality is hit-or-miss. Worth $5, though, even if they might not last that long.
Went with Koolance for the blocks, but that is just my personal aesthetic preference at 9:30 PM EST.
Case Labs M8 <3
Seasonic X650 PSU; fantastic, hits 80+ Platinum in cold testing, has stunning load line reg, great ripple, and overall, is a fantastic PSU, more than enough for if you want to throw another 780 in there.
Mpower Max instead of MVIE. Watch this to find out why I highly recommend not buying that board. Asus can make some, and does/has made some great stuff, but damn, the MVIE was a flop.
16GB, in 8GB sticks for the most expansion room, of G.SKILL Sniper 1866mHz CL9 memory. Fast, cheap, low profile, graet looknig; what more could you ask for? More of it.
256GB ADATA SX900 SSD, as explained earlier.
8 XSPC 3/8 by 5/8 compression fittings, with a black finish. 2 for the integrated res/pump, which is an Alphacool 12V pump, 2 for the CPU block, two for the GPU block, and two for the radiator. There is also a Bitspower G1/4" plug, because the pump/res combo has 3 ports, not 2, and does not include a plug for the additional thread.
Lastly, 10 feet of Primochill Advanced LRT 3/8 by 5/8 clear tubing. Note that I didn't include any coolant. If you want a colored loop, I'd get colored tubing, and use distilled, di water as the coolant, because the mixed coolants are both expensive, not as long-lasting, and can gunk up, clog blocks, and stain tubing.
If you have any other questions, feel free to ask.
