Just get the HD4850/HD4870, they cant be beaten on price / performance ratio.
Would you really spend twice the amount to get only 10% more performance by buying a GTX280?
The drivers are very early in beta stage and its not even the final BIOS for the HD4870X2 so I wouldnt judge the performance of the card based on those reviews, it will be better. You can also easily overclock the HD4870 to get the same performance of the GTX280 ;)
FSX is terribly optimised so I wouldnt base a card purchase just on that, that would be kind of stupid considering how highly priced they are.
The GTX280 has already dropped $150 and its still not competitive, dont ever think it will go below $400 anytime soon.
If you havnt got much money dont go and waste $449 on a GTX280 when you can get very close performance for $150 less :)