This is the last generation of the fat-tube beer-can Cannondales right? Single-speed will be tough to do because of the vertical dropouts. These bikes may have been a bit on the "too stiff" side of the design. Great for smooth crits, but on road-races or mountain downhills, it beat the crap out of me! My downhills were actually slower because of all the skipping and sliding it did around corners.
Personally, I'd build it up as a crit bike, add a carbon fork for some shock absorption. Build some lightweight tubulars and have some fun!