4870 or 9800gx2 or XFX GTX 280

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!

High welcome to the forums,

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.
Mactronix  

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.

Flight Simulator X is one billion percent CPU limited. I play that game and got 4 FPS increase going from an 8800GT to 4850 Crossfire. Even the GTX280 sees minimal gains.

Keep the 8800GTS, it is an awesome card and upgrading is pointless.

Good Luck! (Oh but if you must +1 for the 4870)

I'd wait for the X2, as much as I love the GX2, the X2 will wipe the floor with it .

IndigoMoss some of the facts you stated are wrong.
Like the 9800GX2 is not more expensive than 4870. It is about the same.
For the 4870, it is more expensive than the gtx260.

The 4870 isnt more than the G260 http://www.newegg.com/Store/SubCategory.aspx?SubCategor... tho the x2 is equivalent