FS2004 is a great sim for the winter months when outdoor activities are minimal.
I use to fly on-line at the http://vatsim.net network, atc was available at the major artcc centers, mostly at night when folks are at home to man the centers.
I remember the days of trying to land, boy!! flying the heavies was the most exciting, trying to land with a cross-wind was just plain dangerous.

There are many add-ons available for fs2004.
http://avsim.com
http://surclaro.com
http://simviation.com
These are all freeware
A good payware(for the intense simmer)site.
http://www.flight1.com
I use USAroads, (found my street), and ultimate traffic, this one adds air and ground traffic(aircraft), these add to the immersion of the sim.

For the really hardcore flight simmer, Lockon-modern air combat is awesome.
Fly the F-15, A-10, and the Russian SU's and Mig's.
This game is processor and graphics hungry so you would need a up-to-date system.
I use a saitek X-45 for my throttle and stick for both sims.

Man!!! if only I paid more attention to school work and not the girls.