Well first if you decide to get new fan'(s) then you would need to make sure you get one that fits with the chassis (body of the compputer, or the box as some peoples call it)
and also the smaller the fan is the more sounds it make, and the bigger the fan is, the less sounds it makes.
as i did, i have option to install a top fan, a rear fan and front fan + fans inside the body of the computer and i also use a cpu fan that actually is large enough to cool down the entire pc by itself (not neccecary for everyone and its bit expencive.)
but what i would do is that double check if the fans on the gpu runs at a reasonable speed, if not change them out then put on some external fans if you got enough space. i recommend that you use enough fans so you can actually feel the wind when ou put the hand inside the machine pretty well.
(inside mine i got full turbulence but for me its neccecarly because of the areas my computer are in)
and also heres a video for thermo paste applying if you need: