Pretty weird that your solution to fix an issue with your AMD GPU is to install an NVIDIA GPU.
Your problem probably had to do with AMD driver's compute mode settings. Possibly, you were using a driver version that required you to manually turn on "compute mode" but then this was automatically done for you because when you installed a new GPU, the AMD driver settings also screwed themselves up in a way that made your GPU run on different drivers (do you have the aug. 23 blockchain driver also installed?).
Most likely he added the older nvidia card to handle the machines display output.
If you're not using onboard video and relying on one of the Vegas to power display output, then you're going to see hashrate drops and will need to lower intensity on the card driving the video to prevent crashes.
Just guessing from experience - though I don't use Cast.
Now if he is feeling adventurous flash them to 64s, tweak the powertables a bit and hit 1950-2000 hash/card