I'm thinking to get the Tamron AF28-75mm F/2.8 XR Di LD, so that it can be my general purpose lens too. I don't think the lens is any worse than AF17-50mm F/2.8 Di-II LD right?
so long it has a motor like your current 17-50mm tarmon, it will work fine on your d40x.
but then again both of your lens will overlap isn't it ?