@bigsnake
The newer, budget-oriented GT2** are a good choice as well, as they are meant to replace the 8000 and 9000 series. A relatively lowend card could run it fine. 4870 is overkill if you're JUST running FPSC X9, but if you game a lot as well, it's not that bad.
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Like BigSnake said, you do not need a very powerful PC to run FPSC X9. I've known people with up-to-date integrated gpus who run it just fine. Here are my recommendations though:
On the nVidia side, a GT220 or GT240 would probably do you fine if you have a very low budget. Maybe a 9800/9600 or even a GT250. If you want though, you could hold off until a month or so when nVidia releases their 'Fermi' cards. Quite possibly a GTX260 as well if you're feeling daring. It's performance is slightly higher than a 4870.
If you're a high-end gamer/enthusiast, I would recommend that you wait until nVidia releases their Fermi/300 series.
On the ATI side, something as low as a 4350/4550 would work fine, but that's also budget oriented. If you have a higher budget, a 4670 would be a good midrange card, as well as the 4770. You could get the 4800 series, but only if you're using stuff like EFX, or just a lot of shaders. You could hold off another month to choose between ATI's lower end DX11 cards.
If you are a high-end gamer/enthusiast, I would recommend you wait for Fermi for the sole reason that ATI will be forced to push prices down on their products. Then, you could probably get a 5870.
