I too have a 5770 and am getting great framerates with clear skies. I'm using *******'s tweaks to an i7 930 @ 4.01 GHz, Nick's setup recommendations etc., shader 3.0 mod, and the latest CCC drivers (11.7). Throw some REX weather in there, and my framerates tank from about 30 down to as low as 9! I've had to turn AA mode to super sampling in order to kill the distant blurries beyond my LOD radius, which is set to default. I'm seeing others with a similar overclock and they're not complaining, and yet searching through these forums I keep seeing "I have a 5770 and clouds are destroying my FPS" posts. I'm beginning to consider upgrading my GPU, and would switch to nvidia based on the track record with FSX alone.