Two identical monitors, or one LCD/LED and one CRT? Also were they both 60hz inputs or 60hz and 120hz? 40fps vs 120fps (constant) isnt as bad as say 30fps vs 60fps but its still a noticeable difference to many people. Maybe not you or due to something you had configured, but to say 40fps and 120fps are just as good as each other just isnt true. 40fps is quite acceptable in some cases but if 40fps was always acceptable then why would we bother allowing more than 40fps?
Here's a few items that can help on your adventure
3dfx 60/30/15 demo
http://www.falconfly.de/artwork.htm (you will need a glide wrapper)
15/30/60 demo using flash
http://boallen.com/fps-compare.html
Someone should make a flash demo that you can enter your own custom FPS rates into and stretch to whatever size you want.. :) Maybe add a "stutter" option that allows you to insert short or long frames or double/triple frames periodically.