Thanks for the replies guys. I had just watched this video
which suggests that you can actually tell the difference between say 200 and 500 fps on a 60Hz monitor. I'm not sure if what he's saying is correct though, what do you think?
That video is perfectly correct, you
can feel the difference and is something I've experienced myself. For the very lowest input lag, have the PC freewheeling (no vsync) with a very high framerate and on a 144Hz (165Hz for the latest models) monitor. Yes, you'll even notice the difference on a 60Hz monitor, although much of the benefit will be lost. Note you'll never get perfectly smooth animation if vsync is off though, but it's a price worth paying if you want the fastest reaction times.
This is because the PC is still reacting much faster to your mouse and keyboard inputs, which is noticeable even if the picture is only ever updated at 60Hz. I feel the video is being quite honest about his experience by saying he doesn't exactly know why this is, but that it's his experience.
You'll notice the difference most obviously on a fast twitch shooter like he's playing in the video.
It's interesting how he claimed that adaptive sync actually adds latency (input lag) at very high frame rates and it wouldn't surprise me if it's true as there is a very small amount of processing time associate with this technology. I've never used this though so can't speak from experience.