I updated my 96 model Super V-1000 which had the original DD50 shock with a Fatty SL air/oil shock I bought on ebay for $150. A GREAT upgrade!
Most Cannondales have "headshocks" which means the shock absorber is located in the head tube rather than in the fork legs. That requires a larger head tube than normal, so you can't directly fit anything but a headshock in there unless you get a reducer to reduce the diameter to a standard size.
Unfortunatley, Cannondale charges your left nut for a new headshock, so unless you've got the big bux or an extra nut, you'll have to get one on the used market. You're taking your chances there, so be sure and ask a lot of questions before bidding/buying.
If you get an air shock (much better than the coil and elastomer types), make sure it holds air for MONTHs at a time. Find out if it has been serviced recently (oil change, new seals, etc.). Make sure there are no holes/tears in the boot. The shock should move smoothly and quietly up and down, and if it has rebound adjustment/lockout it should be working in all positions. Make sure the brake mounts are compatible with your brake/wheel. Some of the headshocks are for disk brakes only.
TD