Well, I finally found 'something'. Anandtech revisited the 3870 x2, and did a bit more testing. They got 50.4 FPS at 2560x1600 on High Quality with no AA using DX10. Using the single version they got 44.4 at the same settings. not too much go go on, but it's a start. No info on the system used, other than the fact that they used the Vista OS, since it's DX 10. That is something that comes into play with FSX, since it scales very well with processor power.
I'm thinking that with my system the x2 will be a good thing
I doubt that I'll run it at that high a res, since my FW-900 only goes to 2304x1440 max. I figure that at 1920x1200 and with a few 'tweaks' to FSX, I should be able to get a constant 40-45 FPS. With FSX, real high FPS isn't as important as having it remain constant, and anything over 35 is more than adaquate.