I read somewhere it was suggested NOT going into the BIOS after installing the new CPU.  I gave this a try and it either worked, or was a coincidence because I now show all 16GB RAM available.  Thanks all for the help.  Going to add a GTX 1060 and it should be pretty much maxed out.
So I have an old Studio XPS 8000 from 2009 that I am playing around with as a side hobby.   I swapped the i5-750 for a i7-870. The Task Manager says 16GB of installed RAM, but 8GB is usable. I swapped the original CPU back in and all 16 GB are available. I swapped to different RAM and the exact same results.  The BIOS is up to date and these old BIOS have very little adjustment. I went into the advanced boot options and made adjustments there with no success.   I'm not sure what else to try and there isn't much info on the internet with a system this old. I can't find out the reason the CPU isn't recognizing the second bank of RAM slots. My only other option would be to try and find 2x8GB sticks and just use the two main ram slots.  I realize this is a longshot question.
Studio XPS 8000, CPU upgrade i7-870, 1/2 the ram usable?