wow well I suggest in the future to use the search feature....this topic has been gone over probably 100 times by now.
Anyhow to your answers:
If you go with a good size intake w/heatshield, optomize TB, and then get a cat-back system you will probably come close to gaining 15 hp at the crank.
Anyhow I would start with optomizing your TB (how to is listed on the site) and then get an intake (open filter, K&N or AFE come to mind as good ones, no reverse cone). Make sure you get a filter that is will flow enough CFM (others on here will help you with that).
As far as exhausts are concerned:
Many catbacks available for the contour and it also depends on whether you want duals or single.
Bnoon has a very simple custom exhaust recipe that you could follow for true duals.
2.0" or 2.25" true duals: dual pipes and cats all the way back with mandrel bends (available from JC whitney) and then resonate to taste (Magnaflow, Flowmaster 40 series, dynomax are some good mufflers).
2.0" pipe for Zetec and duratec naturally aspired
2.25" for 3L
True duals will give you the most power for a dual setup, and probably biggest gains overall.
