1. I was just thinking to remove the 2 four gig completely since 32 gig is way more than the 8 I had. 2. Any reason to keep the two 4 gig sticks attached? Other than to just have a bit more?    1. That's what I would do. Remember, you want Dual-Channel Operation. Hopefully (old) XPS-8910 can handle 16gb DIMMs. Due to the machine's age, I would have probably gone with 8gb dimms x 4 (all matching).  2. Not that I can think of. 16gb total would be enough for this machine unless you are running VT. You are pretty-much limited to Windows-10 (64 bit).
My XPS 8910 came with two 4GB RAM cards. I just bought two 16GB RAM cards from Crucial. Question is do I just add them into the other 2 slots (black) or do I remove the two 4GB cards in the white slots with the new ones? 
XPS 8910, adding two 16GB