For realism I'd go for a DCS title over Cliffs, you can download DCS World and try out the Frogger for free, it's more basic than the payware aircraft but still involving. The payware DCS aircraft are a work of art (check out the DCS threads around here) but not the sort you can just get in and taxi/take off without reading instructions beforehand. Cliffs is pretty, but there are (were - the last I played was a couple of months ago) still some bugs* but overall it was still enjoyable and my PC is at the lower end of the running spectrum.
*I'm not saying that DCS isn't without it's bugs, but they do work on them seemingly more often.
You may also want to try Rise of Flight, like DCS you can download a demo version (no longer timed) before plunging for an upgraded version or just the odd payware aircraft.
Links to all sites are sticked in the Flight Sim Hangar thread.
