Personally, I'd buy F40fd. It has image quality close to the mighty F30/F31fd; unlike the F50fd. And IS, though sounds appealing, will offer minimal benefit at 3x optical zoom. Plus the F50fd IS doesn't seem to work so well as per review by DPreview.
Nikon D90, D80
Nikkor 16-85mm AF-S DX F/3.5-5.6G ED VR, Tamron SP AF 28-75mm F/2.8 XR Di LD Aspherical (IF) macro, Nikkor 50mm F/1.4D, Nikkor 50mm F/1.8D, Nikkor AF-S VR 70-300mm F/4.5-5.6G IF-ED, Sigma 105mm F/2.8 EX DG Macro ||| 2x SB800 | SB600 ||| Manfrotto 190XB