Gaining and keeping 20 lbs is a lot to expect out of any single cycle. It is possible, however unlikely.
That said, Test and Dbol is a good first cycle. Deca and Dbol is a good combo, also. Just don't go cheap and skimp on your ancillaries. You had better have HCG on hand in case your balls shrink up, (never happened to me). Also, concentrate just as much on your Post Cycle therapy together as you do on the Anabolic steroid portion of the cycle. Check out Radar's PCT sticky at the top of the page, lots of good stuff there.
Good Luck.