[Ethereum] How to mining using GPU in private chain

With ATI GPU, your CPU is used heavily during the dag creation phase but after that, there's almost no impact on CPU except the whole computer heat generated. Also disk access and network are lightly used during mining. I think you just should try and see it by yourself but if your computer is well cooled, you should be ok.

For the best efficiency, you probably need to run 2 instances of ethminer: one with flags that tell it to use only the AMD card, and a second with flags that tell it to use only the nVidia card.

Genoil's ethminer works with CUDA if your nVidia card doesn't support OpenCL or if you want maximum performance out of it.

https://github.com/Genoil/cpp-ethereum

As for the flags you need to use, read the manual. Pick the default flags they suggest, then add flags to choose which GPU to run.