Optix requires a GPU with compute capability 5.0 and higher, and a driver version of at least 470, according to the Blender specs.  Optix is currently ONLY supported by the RTX cards in Blender.  There is speculation it may become available for other Turing-based cards like the GTX 1660 Ti, but currently, Optix requires the RT cores found in the RTX cards to use Optix.  The GTX 1660, although it may have the Turing architecture, does not have the RT cores.

Optix is much faster than the Cuda backend in Blender, and Cuda was much faster than the OpenCL.  However, now AMD users have a new option for GPU enhanced rendering, so that speed has been much improved.  I don't know how much speed improvement Cycles X will offer to Cuda-based cards, but Lobo has suggested it is significant as well.  Still, if you can get an Optix-enabled RTX card, you should see the greatest speed enhancements because of the extra RT cores.

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