yeah...you can definitely upgrade the fork on that bike...but why the hell would you want too?
cannondale's system integration thing that they have going produces some of the lightest, stiffest, and fastest components out there. your headshox weights 2.8lbs... you would be hard pressed to find any suspension fork out there other than the new manitou mrd R7 and the reeallly expension lefty that weigh that little. i own a R7 platinum from last year, and the headshox is just as good.

while your headshox may only have 80mm or travel, its a good 80mm...with forks its all about the quality of the travel instead of the quantity. i have found headshox to be one of the smoothest and stiffest forks out there. go read about them on cannondales website and i think you will reconsider...