Best Aircraft Texture format For FPS in FSX???

[Brett Hill 2]

I want to make a certain aircraft in my collection give me better frame rates, I have got a program called DXTBMP to help hopefully convert the aircraft textures into a more frame rate friendly format for FSX + SP2/Accel on a vista computer in DX9 mode. (have made sure that it is the textures that are giving me bad fps as I have taken them out completely and it gives me much better FPS).
But in not sure what the best format to convert them to is if I want optimal FPS, Any Ideas?