This is one of the top questions we answer on a daily basis at CannondaleExperts.com. We have a post about matching up mountain bike headsets here. But there’s a lot more information there than most people need.
If you have an SI headset, meaning the Lefty/Headshok bearings (HD169) fit directly into the frame, and you want to match up a 1 1/8″ to 1.5″ tapered fork, you need the KP205.
Search for KP205 on CannondaleExperts.com
Bikes with SI headtubes include Scalpel 29er, Flash, F-Si, 2011+ Jekyll, Trigger, Rize, RZ, Rush Carbon 26″, Rush 29 and many others starting around 2009.