Reference or ACX?

you 15, I'm 16 and not even I would want to do custom water cooling. Too much of a risk at such a young age cause if you screw up that's it not like you got a awesome job to pay for it back. so scrap that.

In my opinion I suggest always going with an aftermarket cooler over the reference one because you have two advantages.
Most decent aftermarket cooler come packed with heatpipes and arrays of the sort which really bring down 'under load' temps alot, however this won't be useful unless you case has proper cooling aswell, well tbh your cause should have proper cooling whether you use reference or aftermarket GPUs.

Secondly, Nvidia's partners create their own PCBs that come packed with this like 12 Phase power and increased voltage limitations. This allows aftermarket GPUs to overclock better while maintaining reasonable temps with their aftermarket coolers.

Warm air will linger in your case whether the cooler pulls in air or pushes it, its up to your own accord of putting an exhaust fan which you SHOULD have.

On the note of adding another card, I still wouldn't suggest putting it in a loop because put it this way. Your gonna spend $300-$500 buying the pipes, blocks, pumps, radiators, pressure fittings and fans. Okay you got your w/c parts now to set it up. Fast forward a week. it's working great woohoo! yay. Now you decide you want to add another card. Okay now you'll have to drain the loop completely, buy more pipe, pressure fittings and another water block oh and let's not forget get some of that extra water cooling liquid, chaching! $200 spent once more. now you gotta fill your system again.

Fast forward 5 years, your GPUs are showing it's age while gaming so you gotta upgrade, you gotta do what you did previously but spend a much larger amount of money and more time and effort.

your solution to this you ask? scrap watercooling idea, I'd suggest just close looping your CPU and selling your current GPU and combining it with the money you plan to buy another GPU with and buy the best single GPU you can.

*Tip, the card your getting doesn't have to be ACX. MSI's TF IV, Gigabyte's Windforce 3x and ASUS's Direct CU ii are also great coolers.

**Edit sure customer water cooling is good, and it looking amazing but it's only ideal if you plan to buy the best stuff to last you longest amount of time before you'd want to do the procedure again and that means a fat bill on your end.

If you have ever had to clean the dust out of a reference design card, you'll never buy another. With any decent amount of airflow in a case, case vented Gpu's are not any real problem that fans cannot overcome. Listen to the previous posts. While WC may look awesome, and work fantastically well, its not something for everybody. Those ppl who do WC take OC performance to its very edge, and beyond, and have the time and resources to do so. And they are dead serious and perfectionistic about everything. It only takes 1 minute leak to render a $4000 pc useless.