2009-05-01 : The three parts of the ability happen sequentially. For example, if you control a creature with 9 power when the ability starts to resolve, you’ll put a +1/+1 counter on each creature you control — and now, since you control a creature with 10 power, you’ll gain 10 life.

2009-05-01 : This ability triggers at the beginning of each of your upkeeps. Whether you control any applicable creatures is checked only when the ability resolves.