I do a little Kettlebell to suppliment a lot of cardio. When I use 2 20lbs bells for the most part. I could probably easily go up to a 25lb bell, but it just really depends on your workout. I tend to do lots of reps so 20-25 is good for me.
There's a good book called "Enter the Kettlebell" that is kind of known as the KB bible. I should read it myself, but I hire someone to figure all that stuff out for me!
http://www.amazon.com/Kettlebell-Str.../dp/0938045695