If you're going to buy a tool specially for this, the little 40W thing with built in vacuum bulb radio shack sells does work fairly well, though don't expect to use it for more than a few projects (it'll get clogged).
Generally for desoldering things that aren't in large planes or have good thermal reliefs, 30W is a minimum, with 45-75W temp controlled being good. For large planes, 50W seems to be a bare minimum, with 75W often inadequate. I had to use the iron plus a heat gun to swap out a power jack on somebody's laptop mobo recently.