Below are the recommended system requirements for the Oculus Rift, which are likely fairly similar to what you'll need to support SteamVR.
— NVIDIA GTX 970 / AMD 290 equivalent or greater
— Intel i5-4590 equivalent or greater
— 8GB+ RAM
— Compatible HDMI 1.3 video output
— 2x USB 3.0 ports
— Windows 7 SP1 or newer
Definitely the GPU
http://www.pcgamer.com/steamvr-everything-you-need-to-know/