I have a Cannondale Quick 6 too and was thinking of getting a Ritchey Comp Cross fork, since it is a carbon fork with linear pull/cantilever brake bosses already on it. Since you want a suspension based fork I would say your options are going to be limited. You could look into a 29er fork, but most of those use disc brakes, so you'd need to convert from linear pull in the front to disc brake, and you'd also need a disc brake wheel. Also, most of the suspension forks out there are pretty expensive, especially given the price of the bike. Perhaps you ought to look into a Cannondale Headshok fork? Either way, make sure there is a lockout on the suspension for times when you're cruising on flat, smooth road.