Right now you have two choices: First Eagles - The Great Air War, and Over Flanders Fields (a multi-part free add-on for Combat Flight Simulator 3).
First Eagles is good fun, and it will run well on pretty much any computer, but it is somewhat generic in that it is developed with the Third Wire flight sim engine which is used for Wings Over Europe and Wings Over Vietnam, that's not necessarily a bad thing, as it is a reasonably good flight sim engine, just don't expect to be wowed by the ground detail, as it is only 'adequate' rather than 'amazing'. If you want the world's most accurate flight model, this is not it, although you do have to get pretty extreme for the flaws to start showing up, and it is generally good fun. TrackIR is a bit of a pain in the ass to set up for First Eagles, but there is very good support for it on the Third Wire forums, and there are tons of repaints and mods available for it too. I would say it is worth having.
Over Flanders Fields is kind of like the GWX for Combat Flight Simulator 3, it's more like a completely new sim than just an add-on. Remarkable when you consider that it is free. Modeling, terrain and campaigns are all good, and it's kind of hard to believe it is based on CFS3, as it takes that sim into a new league. Shove all the detail sliders up on it and you will need a fairly decent computer, but if you have TrackIR and a reasonable fast computer, it really does take some beating. and if you have CFS3 (available for about 7 quid), then you can't argue with the price of Over flanders Fields, because it is free.
There are a couple of WW1 flight sims in the development pipeline, but don't hold your breath waiting for them, as ventures like that fail more often than they succeed, so they may never materialise.
Here is a link to a couple of vids I made on Youtube, one was made with First Eagles, the other with OFF in CFS3.
Made with First Eagles:
Made with OFF in CFS3:
Here are a couple of screengrabs of a plane in First Eagles, this is one of my repaints for it:
Chock