If you've got $850, there should be an i5-2500k in there. You can easily overclock it to 4.4 GHz for about a 40% speed increase.
Since you're set on that gigantic case, I got a full ATX board, with heatsinks on the VRM and an 8-pin power connector. Compared to a budget mATX H61 board, this one can overclock, should run cooler and be more stable, has more RAM slots in case you want to add memory, and has another PCI-E slot if you want to Crossfire later.
RAM: G.Skill or bust.
I chose that PSU because I was having trouble finding something with cables that would span that gigantic case of yours from a bottom-mounted position. I actually use this exact PSU, so I took the side panel off my computer and checked to make sure the motherboard power cable would be long enough.
With shipping, I think it comes out to $855. There's also $25 in mail-in rebates.