IMO you might just be better off getting a whole new bike for not much more than getting a whole new groupset. Those old Cannondale frames are generally sturdy, stiff and accelerate like a rocket, but ride like a wooden horse. $800 bikes are pretty decent these days with more gears, better brakes, brake/shifters (If you have an interest in them), nicer riding frames and current parts that you can still find some spares for. You may also still be able to sell most of the 600 parts and recoup some of the cost of a new bike.