Since you're already familiar with the AMD setup the AMD card may be a good idea.
That's a nice performer in all regimes and shouldn't come with a downside. For FSX the Shader 3.0 is recommended though.
With Edwin running the same card and system, he may also help you on some settings then.

Different prices may arise through the package details like bundled games and stuff like cables. Some manufacturers also offer a different cooler setup, sometimes better or at least quieter than the default. Take a card with at least 1GB (are there smaller 6870?) and you're good to go.