If I'm using my gas grill (adding charcoal and wood) and wanted to try to get different flavors out of different wood types, is there a guideline for which woods work better for fish/meat - Hickory is a strong flavor and works great with meat; Cherry works good with chicken...Any other ideas? 
I'm not sure if your question refers to smoke chips or plank cooking, but I really enjoy salmon cooked on cedar planks.  I got a great deal on the cedar planks on Woot a while back, but you can actually buy untreated lumber at your local Home Depot/Lowes/etc and cut it to size yourself. 