I can defintily reccomend falcon 4.0: allied force! With Lock On I'm sure you've heard the number of complaints about the instability and the need for a high-end system, but in Falcon, you can have gorgeous scenery, clouds, planes, and FRAME RATES, on a medium system with no problem.
The learning curve is HUGE however if you choose to go for full realism, and will probably take you much longer to learn to fly the F-16 than it would for the PMDG 747 and planes like that. It may seem limited that it only has the one plane, but believe me, it is FULLY simulated.
I do not currently have it installed on my computer, and never did really get a good handle on the radar/missile system (on maximum realism), but that is mainly from my lack of hard drive space
. If I had the room, I would definitly still be playing it today!
So for the price, the stability, and the realism, I reccomend Falcon 4.0!