I think that wearing a tie, and a long- or short-sleeve dress shirt, should depend on just how hot and humid it will be. Though either way should be tolerable if your church has a decent air conditioning system.

A black vest will dress up your appearance, but will increase heat retention. However, you don't have to worry about the length of your tie or wear a belt with a vest.

J. Higgins sells a black wool Argyl vest for $65:
http://www.jhiggins.net/sf_catproduc...Q17UTNL7NBUTSD

Celtic Corner sells a polyester kilt vest, which should be cooler than wool, in several colors for about $37:
http://www.celticcornerstore.com/cat...spx?cat_id=135