My suggestion...
Get any motherboard you like, so long as it has at least one PCIe 8x or 16x slot. Go with integrated graphics so you can keep that slot (if there's only one) for a hardware RAID card. Go with any case you like.
Get a good hardware RAID card. If you want to do RAID5, look at the Areca cards. For 8+ drives, this will be more expensive than the rest of your PC, but worth it - RAID5 parity calculation will be completely off loaded from the CPU. I would get one with an external SAS connector, and run it to this external hot swap enclosure:
http://www.span.com/catalog/product_...ducts_id=16416
SAS is backwards compatible with SATA; you can still use SATA drives in this array.
If you really want everything in one case, then look at SuperMicro cases, or get some removable drive caddies that let you put five 3.5" drives in the space of three 5.25" bays, and a case with lots of 5.25" bays.
I would seriously suggest you mirror rather than stripe your OS drives. You really don't need the speed, and with RAID 0 you lose the whole array when one drive goes. I gave up striping my Raptors when a bug in my motherboard raid driver lost me data, and with lots of RAM in my workstation the paging file hardly gets touched, so RAID0 is overkill.
If the purpose of this box is to serve big files quickly to several client pcs simultaneously, multiple RAID1s will perform better than one big RAID5, but obviously you only get 4TB useable space from 8 drives (vs 7TB with RAID5)
cheers,
Aitor