Do you know if it is a bad graphics card? I have seen some issues with the GTS that have something to do with Direct X 10, but this is the GT, so probably different.
The card provides less performance than a 7900GT, the DX10 core makes no difference.
You would be faster on a 7900GT 256 than a 8600 256 if that gives you a perspective.
A 7950GT 256 would be even faster than a 7900GT mainly because it provides 24 pixel pipes over 20 from the 7900 and if you select the right one, a faster core and memory speed. The 7950GT 512 is even better for FSX because of the 512 VM but is difficult to find right now on PCIe. There are plenty of AGP versions available but not PCIe.
If I was going to buy a budget 8000 series card, it would not be anything less than the 8800GTS 320 with a 550+ core speed and a 1700+ memory speed with the 320-bit memory interface. It also has 96PPL combined shaders which is really about 36-48 but far greater than 24. The more PPL the card has the more efficient and better it is in 3D.
With that you would pretty much chew through FS9 and be able to run FSX much better.