David Martland The HP Z840 does not support GeForce SLI configurations.
It supports an SLI with these specific Quadro GPUs:
M4000, K5000, K5200, M5000, P5000, K6000, M6000, M6000 24GB, P6000
Unfortunately, these cards are not great for gaming.
However, they do work well for Auto Cad and other Professional applications.