It depends on your goals.....
A. If we are talking CLCs, I'd say no. They don't do anything better than air coolers costing less and are as much as 12 times as loud....which you state is the oppsitye of your goals.
B. That doesn't include AIOs that are not CLCs. The Swiftech H240-X / H220-X compete well with custom loops
1. Water cooling using a custom loop is fun and an enjoyable experience.
2. Aesthetically it can "look cool" but that depends on the individual ..... flexible tubes going every which way offend my sense or "order".
3. Not necessarily so
here we see the long standing Holy Grail of CLCs water cooling (Corsair H100i) scoring 73C and 68 dbA get it's tail kicked by the much cheaper Noctua NH-D15 air cooler which kept the CPU 2C cooler while being 1/12th as loud ! So when we are talking CLC type water cooling, the one that generally compete with air coolers, do so be being extraordinarily loud. Yes, you can use slower fans but thermal performance is proportional to rpm.... as rpm goes down so does the unit's cooling ability.
As for the 2nd set of questions
1. If a CLC, then no. An AIO OLC like the Swiftech will give you lower temps at the same noise level but not by much....3C cooler at < 40dbA
2. The 820 supports
Top: 2x 140mm or 3x 120mm
Rear: 1x 140mm or 1x 120mm
Bottom: 2x 140mm or 2x 120mm
Pivot: 1x 140mm or 1x 120mm
A Swiftech 240-X will fit on top.
A Devils Canyon CPU and twin 970s would produce about 490 watts, 60% of which will need to be handled by your radiators.....294 watts. Each 140mm at 1250 rpm will do about 82 watts; so (2) 280mm 30mm thick rads gives you 328 watts
I'd approach it in steps:
1. Install the H240-X on CPU on top of your case
2. When ready, grab a waterblock for your GPU along with some extra tubing and 2 barb fittings
3. When ready, gran a 2nd GPU, waterblock and a 280mm radiator on bottom along with some extra tubing and 2 barb fittings
3. As no one **needs** a computer, I'd have to say no. At 1250 rpm you will definitely be quieter. You **might** get an extra 0.1 Ghz on the CPU as compared to say a D15. You won't get anything outta the GPUs OC wise but they will run at about 40C instead of 80C when OC'd to max and will be much quieter.