The crack will most likely spread, and the exhaust note will be louder. You will also start smelling it in the cabin eventually.

I would pick up a Helms manual (www.helminc.com)...its real useful if you want to work on the car in the future as well. If not, the install is pretty straight forward. Remove the bolts that hold the heat shield to the manifold. Remove the nuts securing the manifold to the head. Remove the bolts/nuts/springs securing the collector to the cat, as well as the manifold support bolts. Pull the manifold out from under the car and replace.