The recommended is nVidia 970 or equivalent. There are options that can make it run smoother on slower graphic cards or at a higher rate.
How can you change the refresh rate to 120 frames/sec? The game reads the current settings from the vr system (steamvr or oculusvr) and if 120 is provided by them it maaaay work. "May" because the CPU could be a bottleneck. Most of the time a frame (we talk about a bigger level) takes less than 5ms which is below 120Hz (8,3ms) but that's on a CPU with 8 logical cores.