The question was, can you make out the difference between 25 frames and 50 frames? I've tried it on 2 monitors. The work monitor had a hard time distinguishing between the 25fps and 50fps but it was noticeable every few seconds. On my home monitor, running at 120hz, the difference was as clear as night and day. Someone decided to take it to the next level and post this link:
http://frames-per-second.appspot.com/
You are able to choose the skin of the objects, add motion blur and set the frame rate to render most of the standard rendering rates like 120fps, 90fps, 60fps ect. Give it a try.
Personally after 60fps I find it hard to distinguish between the frame rates. Oddly enough I can see the difference between the 90fps and 60/120fps.
Whats the highest you can go before you can't see a difference? Maybe Ron or the mods can add a poll for it.