Well I'm using Nvida stuff since some (chip) generations for FSX and I think that older AMD cards (which were ATI) may have lost some fps over Nvidia, but a modern 6870 is a fine card and should not lead to any problems or fps downsides if I remember the forum tenor correctly.
Just make sure to use the Shader 3.0, it really helps the AMD card with the clouds.

You already know the driver menus, you know the tools. Slight overclocking is included in the driver itself and you can also adjust the fan there. Also, when playing some other games, your machine is in a different class with the 6870, so you can play the latest titles in full detail there.