First, for storage, RAID 6 with only five drives seems wasteful to me. If you add a sixth drive, you can stripe three mirrors together for RAID 10. That gives you better performance on write, an 80% chance of surviving two simultaneous drive failures, and a 40% chance of surviving three. RAID 6 can survive two failures, period.
You then need a big case with room for all the drives, and a proper 8-port RAID controller. Depending on how secure you want to be, you might want a cache battery as well, or just turn off write caching on the controller. In general, I recommend LSI.
Beyond that, the rest of your requirements come into play. How many graphics cards? Sound card? You need a motherboard that will fit the cards you require (don't forget a slot for the RAID controller), and finding one can sometimes be tedious.