Just a quick update!
It took me a while (mostly because I almost had no spare time), but I finally managed to update my system. I bought a Noctua NH-D14 cooler (which performs better than the liquid coolers I could afford) and changed my motherboard to an ASRock Z77 Pro4, in order to perform a better overclock. Now the CPU runs at 4.6 GHZ and temps never exceed 60c.
I also managed to obtain a good balance between performance and graphics, building a prepar3d.cfg file from scratch. Now the sim runs smoothly and I'm very happy with the result!
I just wanted to thank again Vic and Kristoffer for their amazing support!
Cheers.