There's IL2, Pacific Fighters, CFS3, and BOB2 (plus a few others) all of which are getting a bit long in the tooth, and none of them are perfect, the IL2 and Pacific Fighters games have great flight models but the enemy AI is questionable in single player and the interface and campaign(s) are somewhat lacklustre too, if I had to choose, I'd say Pacific Fighters had a slightly better single-player campaign.
CFS3 got slammed by a lot of people, but I personally quite like it, the flight model is pretty good (not quite up with IL2, but close enough to be okay). CFS3 is WAAAAY more 'moddable' than IL2 (which apart from reskins is pretty much locked as far as modding is concerned), with CFS3, there are lots of free add-on mods too, notably the WW1 revamp, Over Flanders Fields, which is excellent and well worth a look, even if WW1 isn't particularly your bag, as it's kind of like the GWX of flight sims. On the whole, I think it's a shame that CFS3 didn't win as many fans as it could have, as it was a brave attempt to 'push the envelope' of flight sims, adding the human element and some nice gameplay features, all of which had the potential to give the sim more dimension. But I guess that's wasn't everyone's cup of tea, funny bunch flight simmers.
Battle of Britain 2 is of course available in revamped form, having had a fairly extensive overhaul, but it still features a clunky interface and an uninviting mode of play, although you can play it as a campaign controller, however, since the outcome is fairly obviously going to be a win for Britain, what the point of that is, is beyond me. It does have some good features, the cockpits are really well done and the 'clickable' switches are fun, if not always practical, the sound too is very good, with the Spitfire actually really sounding like one on the fly-by view. But the flight model is almost unbearably over-sensitive, and fails to take into account the fact that it is supposed to be 'simulating' flight as opposed to 'emulating' it. The problem being that the behaviour is indeed very much like a real aeroplane, but not easy to use since we are not actually sat in a real aeroplane with all the sensory feedback and peripheral vision etc which makes it possible to fly a real aeroplane. Expect to crash your plane a lot with this sim, even on what appears to be a perfect three-point landing.
Some other outside choices (if you can find copies), which although quite old, offer some fun are: European Air War, Luftwaffe Commander, B-17 and Fighter Squadron: Screaming Demons Over Europe.
European Air War has some great fighter squadron simulation and an interesting campaign, although the flight model is not spectacular. Luftwaffe Commander is long in the tooth but has a fun campaign starting in the Spanish Civil War. B-17 has the opportunity to fly as an escort fighter and is therefore quite good fun and varied (besides being a good game/sim there are still plenty of mods about for it too). Similarly, Fighter Squadron has some good mods about for it if you are prepared to search the net and has some impressive damage modelling and lots of great aeroplanes including the Lancaster and B-17 as well as lots of fighters and fighter/bombers.
All of these however have somewhat dated graphics, apart from B-17's which still stand up fairly well, even if they are a bit spartan by today's standards. However, they do all at least have the virtue that you could probably run them at really high detail settings with no problem whatsoever, although some might not like XP/Vista.
For a modern sim of 50s to 70s jets, the obvious choice would seem to be Wings Over Europe, but it is a 'survey sim rather than a study sim' in that although there are many aeroplanes available to fly, they are not simulated with any obsessive accuracy. Radar and missiles are all there, but they are somewhat generic, as are the flight models, which although adequate, are hardly breathtakingly realistic. Nevertheless, it's a fun game with interesting campaigns and some challenging enemy AI and there is a healthy modding community for WOE, with many free extra aeroplanes available on the net, including stuff like the BAC lightning and obscure models from the F-86 Series. It is also well serviced by the developer, who is easily contactable and willing to talk to you on forums, which makes a refreshing change these days! Another plus is that WOE is fairly friendly on lesser computer systems, which makes it very fluid even with the graphics turned up, although don't expect the ground to wow you with its realism, as the view is nothing spectacular, however, the clouds are well done.
So there's a few ideas.
Chock