it's all a matter of finding the right balance for what you need
the highest end card available today, will probably force you to upgrade also other parts of your computer as well, just to support itm and even then, it's arguable whether it will be able to run FSX on ultra-high settings (i dont think theres enough computational power in the world for such a task

)
however, FSX can be enjoyed just as well with a medium-end card, which can be found a lot cheaper, and won't give you too many upgrading troubles... but with graphics turned down and avoiding hi-res areas of FS
a cheap card i wouldn't recommend, since you will have stutters all the time in the game and will be probably wondering why you didnt spend a little more for a better GPU
anyways, my 2 cents here
Cheers
HarvesteR