I have some aging Core 2 Duo systems in need of an upgrade. The motherboard combinations are varied so I'm wondering, in a broad sense, are the Core 2 Duo procs interchangeable for Core 2 Quads or do I need to track down each individual motherboard model to check for compatability?
It would depend on the chipset. Figure out the chipset model and that should tell you what's supported and what's not.
From that, I looked at the wikipedia page for Intel chipsets and you can see that there are chipsets which support Core 2 Duo, but not the Core 2 Quad. You'll have to figure out which chipsets are being used in your motherboards and see if it's one of those which does support both.