sure, you can replace the 120mm fans on the sedon. push/pull can help as well.
as for replacing the fans with fractal design r2 silents.... i gotta tell you i'm not a fan. You see radiator fans are different from case fans. radiator fans require something called "static pressure" to be effective. As a result you don't see any system builders putting those excellent noctua fans on their radiators either. those low noise fans rarely produce the type of static pressure you'll want on a radiator.
generally the highest regarded radiator fans out there are the scythe gentle typhoon ap15... they aren't dead silent, but what noise they do make is very low frequency so blends into the background better. furthermore they produce...