Battlefield 1942 and Battlefield Europe: WW2 online(MMO'ish and pay-per-month game, which is the only thing that held it back), were both good WW2 online flight combat games, and I hear Il2 is good too but never got around to trying it.
Battlefield 2 is great for modern jets and helicopters, but generally the jets fly too fast for the map sizes(you go into the edge of the map in seconds, not even to mention afterburners). I feel like I'm in one of those machines that NASA uses to test pilots for what kind of G-force they can handle - because you're basically flying in circles at lightning fast speed and you're going around and around the map so fast you hardly have time to target anything. I don't have a problem with the jet speed, just wish they would have made the maps bigger.
All of these are very old by modern standards though if you're looking to test out a new PC.