I prefer ATI, simply because it is what I have used since I first started PC gaming many years ago, and am comfortable with them, always managed to find an ATI card in my budget that works well. Right now, their 5xxx cards are the only DX11 cards on the market (vista/7 only) and it is unknown what pricing strategy/performance level Nvidia's fisrt DX11 cards are bringing to the table.
FSX is a CPU heavy game, and relies more on clock speed and core count for fps than GPU power, though a good GPU would be needed for the larger resolutions you are needing.
A single 5850 would probably be the best choice, as it far outperforms a single 4890, uses a LOT less power than 2x 4890, is DX11, and would be a huge step up from your old card. Single cards avoid the complications of driver support issues that crossfire users sometimes encounter. You always get the use of your graphics with single card solutions.
If you MUST go crossfire (understandable
) then 2x 5770s would probably be the best bang for your money. They perform similar to 4870s + DX11. Plus the price of 2x 5770 is only a bit more than 1 5850, and for titles that fully support crossfire, will match or beat a 5870 ($400 dollar current top single GPU card)