it sounds like you need to do a lot of research on this subject before jumping in with both feet. YOu'll be very happy with the D300, from other people's opinions who are wedding photogs. You can do a lot of reading at a bookstore, buy the books online, and you could join
www.digitalweddingforum.com for free for a ton of info on wedding photography. This is an area that you can not take lightly.
Better yet, would be to second shoot for someone else. YOu should know how to shoot in RAW for best maximum quality of your images, but that could be debated, but if you are not experienced in church photography, then it could save you.
Good luck