afaik flight simulator x has only civilian aircraft, so doesn't really fit the describtion of "modern Flight Combat Simulators"
Also I think aces high 2 is a WW2 game, right? Again, not exactly "modern Flight Combat Simulators"
I'd suggest Falcon 4.0: Allied Force. Definitely the best flight sim I've ever played. Also heard that Lock On: Modern Air Combat is good but I have never tried it myself. (mostly because after Falcon I could never imagine myself using the keyboard to do stuff) But I think that I will give Lock On a try when I get the time to.