1. Hard to say. To be entirely sure you will need the dimensions of the finished motherboard housing all components, or better yet, Google the maker and name of the case and see what other people say about it.
2. Get a motherboard that supports multicore CPUs and PCI-E graphics cards in SLI or Crossfire. There are lots of brands, so people here or elsewhere will be able to give you good reviews.
3. I swear by NVIDIA, and either a 8800GTX (I have it in 1GB) or a 9800GX which is actually dual-core already.
I would recommend going to an online computer shop and stealing the list of their top gaming rig's components. Then you can try to get those parts cheaper, if you want, or just buy the thing already and start gaming.