The most obvious solution would be Apple's own Logic Express or Logic studio. Both have just been upgraded to version 8 with a much more friendly interface and a huge price drop. If you haven't tried it already Garageband 4 (part of iLife '0

is really easy to start recording with. Many folks still use it to sketch out ideas before moving the project to Logic. It can now change pitch or tempo part way through a project although you can't change the key signature as best I can tell.
Logic 8 (Express or studio) offer more advanced scoring, editing, mixing and and huge number of more advanced software instruments.