I'd echo what TR4x4 said. I did it the wrong way round, got a d700 then FX lenses, i'd suggest getting FX lenses (24-70, 70-200, 50 or 85) first. They'll still be fantastic lenses on the D300 and then up to the D700. Is the lack of ISO performance because you havent got f2.8/f1.8 lenses?