Believe it or not, I would say that Falcon 4.0 is still the best air combat
simulator out there. It has remained supported and upgraded many times by
second party devlopers. Do a Google for it and get the updates. Some of them
are quite large. The entire sim has really been redone and done with
cooperation from actual F-16 jocks. The new upgrades also use a lot of
system resourses and will tax even the quickest system with all the bells
and whistles on. AFAIK, all the upgrades are free to all. The work is
remarkable.

Try looking at the Falcon 4 forum on www.frugalsworld.com - big F4
community there; they'll be able to help you pick out which patch to use
and help you get it up and running. Also, one of the third-party
developers is freefalcon (www.freefalcon.com); the other main one is
SP4/BMS, though I'm not sure what their webpage is. There is also a new
commercial Falcon release (Falcon 4: Allied Force)