Either cpu or gpu but I think at this time it's only for Cuda enabled graphic cards. Here's a video that I find quite interesting: http://www.blenderguru.com/videos/introduction-to-cycles it's quite long but it's the best I've seen so far on Cycles. If you can, the best part start around 15 minutes into the video.
Blender removes vertex normals so not so good for importing MoI objects. This has been discussed before and Blender.Org have no intention of rectifying this major error.