I would suggest before taking any other steps, Read the FAQ section on the site which will explain everything in details for you. Then it will give you an idea of how you should go about planning your system. Also figure out what you want the carpc to do for you, this will also help you in your quest.
I wouldnt suggest you go with dual hardrives, just got one nice sized one. Heat will become an issue as well.
As far as your processor goes that will also generate some heat so unless you have a cooling solution think a little less horse power.
i used desktop parts for mine, Im in the process of building a slim plexiglass enclosure for everything.