Have a look at this link:
http://www.tomshardware.co.uk/radeon-hd-4870,review-31046-6.html
Ge
force 8800 Ultra appears to be the best.
If your on a budget I'd go for the Radeon 4870, you wont miss 5fps, and would be saving a good USD 300.
If your money is no object, Id go for Dual GTX 260's or GTX 280's (theres not much of a difference between the two). Possibly check if FSX supports Physics Aegis. Though last I heard FSX does not support physics cards.