You have a long journey ahead of you. I am working on an entertainment PC plan myself. To address your immediate two questions.
Media Center cases are pleniful, just do a google search and you will find everything from the $120 plain jane to the pimp daddy Thermaltake VH2001BNS which is what I am thinking about using:
As far as motherboards, onboard raid 5 (sata) is common now adays. The cheapest one I could find is the ASUS P5ND2 for $35, but you can not just buy a motherboard and slap parts on it. You need to pick your processor and video card at the same time to make sure they all match up. pricewatch . com is a great site for computer parts shopping.
Be sure to look at an HDMI video card with audio out so everything is over 1 cable.
Keep us updated with your progress.