Thats a random tug of war since 4890 and the 295 GTX are in 2 completely different leagues.
I say the 295 GTX isn't worth the money, your better off with the 4870 X2 if you need a card with that power.
I think flight sim X is just CPU bottlenecked more than GPU.
If its just for that game, I'd grab a new CPU. I would say that the 9800 GTX+ is enough till next year IMO, but if you really want no headaches the 4890 is def the card of choice ATM.
No questions. p/p