Of WW2 flight sims, IL-2 Sturmovik is by far the best of them all. Dont confuse it with the new Il-2 Sturmovik: Wings of Prey (or something), it shares only the name with the "real il2": this ones originally a console game, and mainly an arcadish flying shooter. Graphics are "better" but its anything but a sim, though the flight models can be tuned to fairly realistic-ish, dont expect it to simulate much more than Pearl Harbor the Movie graphics. It doesnt.
IL-2 Sturmovik 1946 something like 10$, multiplayer is free and there are about a thousand players online on weekend evenings. And a team still makes official updats to it, one coming out in the next few months. No DRM either, available both from Steam and as a DVD copy. Compatible with Vista and W7.
For multiplayer use the HyperLobby lobby software, its 100% free, by Jiri Fojtasek, there: http://hyperfighter.sk