My wife has already agreed: do I still need to go to court?

Yes. The existing wife's consent does not replace the permission of the Syariah Court. The law requires the court's written permission beforehand in every polygyny case (for example Act 303, s.23). However, a wife's consent given honestly and voluntarily usually makes the process smoother. The court will still summon the wife to confirm that her consent is voluntary. The judge must also be satisfied with your financial ability and justice. A written letter of consent may be attached, but be prepared with complete financial documents as well. See polygyny procedure.