What frame rate does the iMac screen have?

Since you didn't specify which model, here are the tech specs for the late 2012 iMac:

http://support.apple.com/kb/SP667

As it turns out, it's a bit more complicated than just fps, since the human eye/brain can only capture about 10 to 12 frames per second...

http://en.wikipedia.org/wiki/Frame_rate

But it can affect the sense of motion/speed, & such...

http://www.100fps.com/how_many_frames_can_humans_see.htm

Size of screen affects fps perception...

Stimulus Area – the larger the stimulus area, the faster the frame rate needed to saturate human perception. Human perception response drops off at 60fps.

http://www.cameratechnica.com/2011/11/21/what-is-the-highest-frame-rate-the-huma n-eye-can-perceive/

So, anyway, you'll see at most 60 fps basically, but since LCDs don't go dark between pixel changes, it should be about as smooth as 120 HZ on a CRT.

Hello, frame rate is different, but the Refresh of most LCD/LED screens are 60 Hz, since they don't flicker like analog monitors, but at 60 Hz you still might get "Frame Rates" in games of hundreds of fps.

The frame rates are also factor of the VRAM in the iMac's GPU. The more VRAM or VideoRAM you have, the smoother the frame rate motion will be.