I'd say 4870, for it is not so expensive as the NVidia cards you listed and the 4870 really performs. Visit this website to get a bigger picture: http://www.3dgameman.com/forums/showthread.php?t=52339
(this page has a graph for 3dMark06 etc...)
Just remember the ASUS 4870 TOP has a 9% performance increase directly from the factory!
It normally isnt a good idea to upgrade just for the sake of one game, but of the cards the 4870 is the better option in my opinion. the thing is though that flight sims are much more reliant on the CPU rather than the GPU.
It would be worth your time going to a dedicated flight sim forum if you havent already.
I do know that Nvidia have a beta driver that has physics capabilities that may help with a CPU heavy title like a flight sim. But also ATI are said to be introducing it as well.
The guys on the flight sim forums would be able to help more than i can though.
Stick around for a bit and some of the other guys that play those games may take a look. I would try a new post along the lines of "Will GPU physics help a lot with flight sims" That should get the flight sims guys taking a look.
I agree with Mactronix, there's a good chance it could be more CPU related than GPU related. Here's the run down on the three cards you mentioned
HD 4870: Unbelievable performance for the money. The GTX 260 is a good purchase now that the 4800 series came out, but it's still more expensive than the 4870 without deals and doesn't support DX10.1, and performs a tad bit slower.
GTX 280: Unjustifiably expensive, not a very good purchase
9800GX2: A higher performer than both of the other cards, but not by a huge margin. Is also more expensive than the 4870, uses almost twice the power, and does not scale as well as a single card solution.
But as Mactronix said, it's not a good idea to upgrade for just one game, especially if it might not be the GPU that's hindering performance.