I think I was just suffering from a recent reformat to install Win7 and not optimizing my machine afterward. Using my trusty www.tweakguides.com
I cut down my services, running programs and optimized my memory and such, then defragged, and it made a world of difference.
Assassin's Creed still dips to the high 20s when a massive number of people are on screen though, but I have been told elsewhere this is normal no matter what with current processors. The overclock would probably help there, I will look into it.