For the 3080s coming out I'm going to go either Hybrid or Hydro Copper and if the latter it will be my first crack at custom water cooling.    My CPU temps are perfectly fine with Dark Rock Slim so I'm wondering if custom cooling is worth it for just one piece, or should you only go custom for both CPU and GPU? 

kougar
It comes down to how much radiator your loop will have. I don't recommend less than a 360mm radiator for GPU+CPU, but you would see a large improvement with more radiator area. I'm using a thick 420mm radiator so there's substantially more surface area over a 360, but the rad gets hot enough I know there's still room for improvement with more radiator surface area. 

a 420 is -exactly- what I had in mind.     So I'm assuming a custom 420mm radiator for GPU only cooling is going to beat their 240mm hybrid, simply because of size?
 
Edit: My purpose for watercooling is a whisper quiet build, not any OC plans.

Sweet!!!   Do you think a reservoir is needed for GPU only cooling?
Also based on what you said, I was eyeing this guy up:
https://www.frozencpu.com/products/25248/ex-rad-713/Phobya_G-Changer_420mm_Radiator.html?gclid=CjwKCAjwnef6BRAgEiwAgv8mQUwIBmdGryfYQH9vXgLujB7GbTjNmaFOMLH3QYKtDMXtLdE2Kf9E0xoCtNkQAvD_BwE
 

yaggaz
Sweet!!!   Do you think a reservoir is needed for GPU only cooling?
Also based on what you said, I was eyeing this guy up:
https://www.frozencpu.com/products/25248/ex-rad-713/Phobya_G-Changer_420mm_Radiator.html?gclid=CjwKCAjwnef6BRAgEiwAgv8mQUwIBmdGryfYQH9vXgLujB7GbTjNmaFOMLH3QYKtDMXtLdE2Kf9E0xoCtNkQAvD_BwE
 

Looks decent to me, but I haven't followed radiators closely. I use a Thermochill triple 140mm but they haven't made those in ages. You will want fans with a high static pressure with that radiator, especially if you are aiming for quiet acoustics. 
 
The choice regarding a reservoir comes down to how often you want to have to open the system up and add new coolant/water. With a reservoir you can wait until the water level gets low before you have to go add more. But you can always skip a reservoir and save some money too if frequent loop maintenance doesn't bother you.

yaggaz
 
Hrm, what to do, what to do.  Go with a Hybrid where I can't control the fan noise, or go with Hydro Copper where I'll end up as obsessed and broke as GTX ;-)
 

It is really up to you. Hopefully this will help...
 
Is your preference as little noise as possible? If it is, go all in. Best to not waste your time with Hybrids as you will most likely find their weaknesses and end up redoing it anyway. Costing you far more.
 
"Raises hands from personal experience"
 
If noise is of no concern get the Hybrid and research what fans to immediately replace the stock ones with. Have that as part of your initial costs. This will allow better cooling and reduced noise (Compared to stock).
 
I built about five systems focusing on air only, and six or seven water-cooled. 
 
This may sound weird......... However, air cooled has always made the ambient air temp higher in the room than water-cooling ever has.
 
Keep in mind I am speaking from the position of running my systems 24/7 full tilt crunching with the 1080TI and my 12 threads on the CPU. The most recent system generates about 1,000,000 points per day. 
 
Now as I say that, I am going a different route on my next build. I am going to WC the CPU but not the GPU. I may relive my own mistakes, but I went down to a Mid-tower for this build. 

GTXJackBauer

yaggaz
 
Hrm, what to do, what to do.  Go with a Hybrid where I can't control the fan noise, or go with Hydro Copper where I'll end up as obsessed and broke as GTX ;-)
 

  
 
In all honestly, if you're not dead set with custom cooling which you should be from the start, I wouldn't bother because custom can have many hickups.  Think of it as flipping a home.  Chances are it won't go smooth sailing but it will be worth it at the end.  

Does it work the same way as flipping a house, like do you smash through the side of the PC with a demolition sledge hammer? 
 
On a more serious note, I suppose I could go the Hybrid route if evga allows the rerouting of the fan cables without voiding warranty?

kram36

GTXJackBauer
Also, I stay away from 140mm rads since their selection of fans are meh ok.  There are better performing 120mm rad fans out there.

Cough ***EK*** cough ***Furious*** cough ***Vardar*** cough ***EVO***

https://www.ekwb.com/shop/ek-furious-vardar-evo-140-bb-2500rpm
 
So it's pretty good.  As a quiet freak I am curious about how loud it would be though.

yaggaz

kram36

GTXJackBauer
Also, I stay away from 140mm rads since their selection of fans are meh ok.  There are better performing 120mm rad fans out there.

Cough ***EK*** cough ***Furious*** cough ***Vardar*** cough ***EVO***

https://www.ekwb.com/shop/ek-furious-vardar-evo-140-bb-2500rpm
 
So it's pretty good.  As a quiet freak I am curious about how loud it would be though.

I have mine (3) on a Scythe Kaze Q fan controller and have them turned down to the lowest speed the controller knobs will go. So I don't know what RPM they are spinning at and they are the loudest fans in my case. They are not as quiet as I would like them to be running them this way, but I like being able to turn the knobs and get faster speeds without going into the bios, plus I can't stand hearing fans ramp up and down, so setting them to "Smart" in the bios would not be good for me. I could run them to the motherboard and see if I can get them to go lower on the fan speed and not be as loud for a test for you. They do move a ton of air and I'm running an extra thick rad.

yaggaz

kram36
 
, plus I can't stand hearing fans ramp up and down, 

AMEN.  That is just the worst, way worse than a louder fan that doesn't change.  That's why I got a fan hub controller for the main case fans.

+2! I think we all are part of the church of the fan controller. I hate building without them.

I use a Aquaero 6 XT curves based on air and water temps but for the most part, my fans don't change too much.  Pumps are static set to keep a constant flow (1 GPM) whether on idle or load.

Surely not a fan of silicon based temps since that's the ramping noise some have issues with.  It would be nice to see curves based on air/water thermals from MB mfgers if they have't yet.

I installed a custom loop (3x 360 mm) on my X299 Dark + 10900X. I "upgraded" from a CLC 280. There is negligible difference in temperature and it doesn't help one bit in overclocking (5 GHz all-core) over the CLC 280. The parts cost almost $800 and it looks incomplete without the GPU in the loop, which will cost another $150. You could buy an RTX 3090, Optane 905p, 4x NVMe RAID, etc. for that money.
 
However I learned something which I did not realize for years - fan speed must be set based on liquid temperature, not CPU temperature. I had connected the CLC 280 fans to the motherboard directly because I though it was better for the BIOS to control the fans. It would ramp up & down and was a constant disturbance. Had I used liquid temperature to control the fans on the CLC 280, it would have been just as quiet as my custom loop.
 
Good luck trying to sell a waterblock flagship GPU in 2 years. Anyone with a custom loop probably wants the latest GPU. If you buy an aftermarket waterblock for the GPU, you can reinstall the air cooler before you sell it.