The minimum spec I recommend is a 6600GT for FS9.
A 6600 is NOT a 6600GT.
There is a lot to understand about video cards. You just can't spout off a card number and expect to know it all.
So there is a good place to start, research the differences between those two cards and you should come to understand a lot more.
The 6600 GPU offers 45% less memory bandwidth and 40% less GPU power than a 6600GT. That suddenly makes the 6600 somewhat less appealing. But they are cheaper. You get what you pay for.