I use kettlebells as well--great workout. I took a one-hour lesson from a kettlebell coach to learn all of the basic swings and jerks. It really gets your cardiovascular going, and it's more of a full-body workout than just weight lifting. Check out 'Enter the Kettlebell.' That guy is considered the guru of kettlebell coaches.
The rule-of-thumb I was told is that a 35lb kettlebell is all you need if you bench press under 200 pounds.