I'm running FSX too. The Level D 767 runs great on every setting turned up fully except the in-game-antialiasing (using nHancer and my GeForce 9600M GT for that), Traffic (64 %) boat traffic (21 % both) and road traffic (0 %, too FPS heavy and terrible looking cars) and scenery and autogen. (Scenery dense and autogen both dense)

I think my laptop works great for FSX.
Using the CS 757 in FSX is not recommended. If you want a CS plane in FSX, you're probably better off with their 727 Captain.
And indeed, there's alot of liveries for it.. Search for "level-d" in AVS*M, FS2004 aircraft repaints.. 50 pages of repaints..
PS: The FS2004: ACOF liveries work with FSX too, as long as they're .lds files, supported by the LDS 767 Repaint Manager. Any repaints you download that is .lds files, are just put in the "Downloads" file and then you use the Repaint Manager to add it. If you download some files that are not .lds repaint files, you won't be able to use them.. I think. I didn't really try.
And Big, FS9 and FSX has some really large differences in how it looks scenery and autogen-wise.. have you for example tried taking off from MDW in both FS9 and FSX, then compared them? You'd need some good addons in FS9 to make it look as good as FSX. I'd go as far as calling FSX next-gen graphics, at least compared to FS9. Then again FS9 has some good add-ons.
FS9 gives you better frame rates in FPS heavy aircraft.
FSX gives you a better looking world without any add-ons, but is FPS heavy.
I don't have any FPS problems, so I say:
*FSX-Forever!*
PS: If you're going with Captain Sim, get the 727 Captain for FSX.
If you're going with Level-D, get the 767 for FSX.