i had to look up that you meant flight simulator x. but toms does a best card for the money every other month.
http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107.html
Which is outdated without nvidia's new fallen prices and without amd's 78xx...
At Amazon.co.uk you can find a great deal for £98!!! Asus 1GB Radeon HD 6850 DirectCU
"Price: £98.99 & this item Delivered FREE in the UK with Super Saver Delivery. "
Even if he asked for a gpu only, I tried to find some numbers to confirm this.
And what I find is that this game is cpu depended but for christ sake it doesnt use your cpu :S.
What I am saying is that its only 2-threaded. A fast dual core cpu is what you can get in order to top it. Or you can take a "K" processor so you can use 2 cores on high speeds (like 4,5Ghz).
Oh, I forgot the source: http://www.simhq.com/_technology2/technology_093d.html http://www.simhq.com/_technology2/technology_096d.html