Andrew,
did you get anywhere with your computers?
We are a company that refurbs older Mac Pro's and a Top Seller on EBay...
From the posts, you may have several issues...
*the memory may be in the wrong slots - from what i read in your posts is that the memory is in the top slots of the memory risers...
Mac Pros like their memory in the bottom slots (the ones closest to the edge connectors)
*your choice of video cards may be problematic as others have suggested - a great generic video card for the 2008s is the ATi XT2600 - very good card and not too expensive OR use an NVidia 120GT (from the 2009 MPs) - you will not get a splash screen with NON-Apple video cards - this can cause you much stress as it may take several minutes for the computer to start with no video output until the last moments.
*you may have old dust and such causing shorts and general confusion on the board - if you have not broken it down we would recommend that you do so and clean it all (compressed air and small soft brushes works great) - including the power supply.
* In taking it apart you will discover things like wrong CPUs on the board (or no CPUs on the board) - we have seen may unusual things...
If we can help, let us know as we help our customers all the time - we may not always have the answer, but we will do our best to find the answer for you...