Add a GPU or add another CPU?

[particlerealities]

I have a Dell Precision T7910, 16GB ram, and one Xeon E5-2640 v3 @ 2.60GHz CPU (with a dual-CPU capable motherboard). I also have an NVidia Quadro K4200 GPU for my display, and an NVidia GeForce GTX 980Ti. I am about to start a new interior restaurant animation project that I intend to render internally (no deadline, so I will be rendering on my own computer).

My question is, should I add a second CPU to my motherboard and render my animation with ADV? Or should I add another single-slot Quadro card, like the P4000? With one CPU, I can only add one single-slot GPU with an 8-pin power jack. If I add a second CPU, I can install another dual-slot card, but I still only have (1) 8-pin plug available.

So, which upgrade path would you choose?

[delineator]

silly question, but I assume you use your GPU in hybrid mode to render, right?

P4000 seems a bit like overkill compared to the rest of your setup - but it really comes down to what your personal bottlenecks are. Also, I would double check the power supply - seems like a third gpu on a prefab Dell might be pushing it a bit far (usually those things are spec'd right on the edge - little buffer built in)

[particlerealities]

I haven't noticed any notable speed improvements when using hybrid mode, but yes, I have tried it. I usually just default to GPU-only since it has zero effect on my system's performance for multi-tasking.

My Dell wasn't maxed out when the I.T. guy spec'd it. I would have preferred to build something ourselves, but they can't do without the Dell warranty for some reason.

The Quadro P4000 (which has a lower benchmark score than the 980Ti) is a single-slot card with 105W power draw and 1792 CUDA cores. It only requires one 6-pin plug. By comparison, the GTX980Ti (and 1080Ti) is dual-slot, 250W power draw and 2816 CUDA cores and requires a 6+8 pin power connection. My Dell shipped with a 1300W PSU, so theoretically there should be plenty of power available, but there just isn't enough available free plugs floating around. If my box shipped with a second CPU, I could easily install another dual-slot card, though it'd have to be a non "Ti" card since I only have one 8-pin left coming out of the PSU.