Do any of you experts know why my MacPro3,1 (early 2008) isn't reading 4 out of the 8GB I installed in it today? The tower came with 2GB, I added 2x1GB sticks a couple years ago, and today, installed 4X2GB sticks, and it now only reads 8GB? I also see one solid red light (DIMM 3) on one of my RAM cards - ...
The red light next to a memory slot means that there is a problem with the RAM card in that slot.
Your system profiler shows that no RAM is reading in the upper riser, slots 3 and 4. If the light is near slot 3, in the upper riser, then likely it's one memory stick that is not seated properly.
Try swapping the pair of 2GB cards between the upper (A) and lower (B) risers.
Be particularly careful (and firm) with seating. It's quite easy to have memory cards that are not seated properly in the slots. In addition, if one of a pair is not seated, then neither will work.
If swapping the pairs doesn't help (and the bad pair swaps to riser (B) ), then you likely have a bad stick. New memory usually will have warranty.
Hopefully, the seating will get all your memory working.