I voted 30-40, but it greatly depends on what I'm doing. I have a fairly powerful system (E6750 overclocked to 3.2Ghz, GTX260, 2GB RAM, 1920x1200) and I even see the FPS drop to 10-15FPS at FARPs/airbases, especially during MP.
Graphically BS, is not that demanding, however RAM and CPU power are important for all the avionics calculations etc.