I'll just add this option here if someone has the same issue:
I decided to try a silent heatsink+fan, and for the second CPU a noisy one
(much much cheaper).
That's a Noctua http://noctua.at/en/nh-u12do-a3/specification
and a Dynatron F661.
Because I found out I can disable a CPU at will:
either in the GRUB config (so every time I restart, I shut down the noisy
CPU+fan),
or with a command that can fire it up or shut it down.
That way, I can get more brainzpowa (with the added noise) only when needed.
Hell, maybe even running both might be silent enough.
I have yet to try, so hopefully I didn't made a mistake on my calculations
for the space needed on the mobo for both.
