D40 will accept manual lenses but there is no metering in any mode - stupid IMO but that's the situation...

To use AI manual lenses you need a D200/300 or one of the professional models (possibly not the D1 though) as these will meter in at least manual mode and I think Aperture as well. Not that the D200/300 can only use AI fitted lenses, earlier non-AI and uncoupled lenses won't fit as the aperture lug round the lens mount gets in the way...