Are you sure you're comparing same games, same settings, no v-sync, etc?
Some tests which should for sure max out the graphics card usage are: Heaven or Sanctuary bench, 3dMark11, Just Cause 2, Crysis/WH, Metro, Vantage High/Extreme, Call of Pripyat ... I'm assuming of course that settings would be cranked high enough to give your card a good challenge, and no v-sync of course.
Telling us what games/settings you're having issues with would be helpful. There's no simple 'blanket' type answer here aside from turn off v-sync and put the settings higher.
Another thing too is that if you had morphological AA forced in the CCC with your Radeon cards, if I'm not mistaken that works on damn near every game ... might've been increasing the load on every game you played and nV has no equivalent so you're not comparing apples/apples.
Edited by brettjv - 4/21/11 at 10:27am