The Collaborative International Dictionary of English v.0.48:

Effectual \Ef*fec"tu*al\ (?; 135), a. [See Effect, n.]
   Producing, or having adequate power or force to produce, an
   intended effect; adequate; efficient; operative; decisive.
   --Shak.
   [1913 Webster]

         Effectual steps for the suppression of the rebellion.
                                                  --Macaulay.
   [1913 Webster]

   Effectual calling (Theol.), a doctrine concerning the work
      of the Holy Spirit in producing conviction of sin and
      acceptance of salvation by Christ, -- one of the five
      points of Calvinism. See Calvinism.

   Syn: Effectual, Efficacious, Effective.

   Usage: An efficacious remedy is had recourse to, and proves
          effective if it does decided good, effectual if it
          does all the good desired. --C. J. Smith.
          [1913 Webster]

WordNet (r) 3.0 (2006):

effectual
    adj 1: producing or capable of producing an intended result or
           having a striking effect; "an air-cooled motor was more
           effective than a witch's broomstick for rapid long-
           distance transportation"-LewisMumford; "effective
           teaching methods"; "effective steps toward peace"; "made
           an effective entrance"; "his complaint proved to be
           effectual in bringing action"; "an efficacious law" [syn:
           effective, effectual, efficacious] [ant:
           ineffective, ineffectual, uneffective]
    2: having legal efficacy or force; "a sound title to the
       property" [syn: legal, sound, effectual]

Moby Thesaurus II by Grady Ward, 1.0:

108 Moby Thesaurus words for "effectual":
   able, achieving, active, actual, adequate, ascertained, attested,
   authenticated, authoritative, banausic, behind the scenes, binding,
   capable, categorically true, causal, causative, certain, certified,
   charismatic, charming, competent, conclusive, confirmed,
   consequential, constitutive, corroborated, decisive, demonstrated,
   determinative, determined, documentary, effective, efficacious,
   efficient, enchanting, equal to, established, estimable,
   etiological, factual, feasible, forceful, forcible, formative,
   fulfilling, functional, go, historical, important, in force,
   influential, institutive, lawful, legal, magnetic, momentous,
   not in error, objectively true, occasional, operable, operational,
   operative, originative, personable, persuasive, pivotal, potent,
   powerful, practicable, practical, pragmatic, prestigious,
   productive, proficient, proved, real, realizable, reputable, sound,
   strong, suasive, substantial, substantiated, sure-enough, telling,
   toothy, true, true as gospel, truthful, unconfuted, undenied,
   undoubted, unerroneous, unfallacious, unfalse, unmistaken,
   unquestionable, unrefuted, up to, useful, valid, validated,
   veracious, verified, veritable, weighty, winning, workable