It has nothing to do with pc133 vs pc100, "sidedness"' or the total number of chips vs memory capacity of the dimm. It has to do with the way the addressing scheme works. Older Intel chipsets require that memory be organized in up to 8 banks (2 per dimm) of no more than 128MB each for a total capacity of 1028. Late model high density dimms organize 256MB as a single bank of 256, rather than two of 128 each. So the chipset recognizes only half or none of it, depending on the whim of the electron Gods...
The Crucial memory configurator will supply the right stuff, but it won't be cheap, or guaranteed used memory with the proper geometry is available from several online vendors...
Older ecc memory may just work, again, it depends on the geometry. Non buffered and ecc dimms can't be mixed, but you can't hurt anything by trying ecc dimms in a non-ecc board. The chipset just won't recognize it.