Yeah, the 512 megs of gddr3 is very misleading because they've slowed down the memory on this card. not really ddr3 if you ask me. Yeah, it's a DX10 card but apparently more about the cards memory which in this card's case was dampened tremendously. As for FSX, I've found that it performed markedly worse than my previous nVidia 7600 GT, which by the way is not a DX10 card. So there. It's a good thing I didn't pay that much for it.
Avoid the ATI 2600 XT if you're hoping for good framerates with FSX.
Avoid it period (regardless of the software you run). The 2600XT was never stellar, it was always low end performance for a mid range price, not worth it IMO.
Finally ATI and Nvidia have some decent mid range cards (3850, 9600GT). Mucg better choice.