we are talking about PC's right? Not laptops? (youre in the Laptop tech support) If you're looking at water cooling, there is quite a lot of AIO (all in one) setup that is ready to go from your local computer store. Those are pretty small and can easily fit inside most setup since the water block is a lot smaller. Your responsibility to check for compatibility on that particular water cool setup.
A few question to ask yourself, do you really need water cooling?
Are you running high temp under heavy workload?
Is my case ventilation the bottleneck?
Are you overclocking?
Can a good aftermarket air cooler do the job? (in most cases yes with current vapour and heatpipe technology)
I am a bit bias and like air cooling better. No point going water cooling because a proper air cooling works very well! Noise levels (depending on your exact setup and Fan noise) is about the same with air cooling plus you don't have risk of leaks, lower maintenance, low wear and tear.
But if you insist on water cooling, figure out how much heat you need to take care off. If there is a lot of heat, get a bigger radiator ( in most cases get as big as it can fit in your case)
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/www.youtube.com/watch?v=uQ1p9QbZ9tc