Nice rig design you have there.

But for five boxing, sorry but there is no other way to put it, its overkill.

For five boxing, a single, higher end graphics card like a GTX275, 285 or the ATI 4870,4890 or 4870x2, with six to eight gig of memory, will eat 5 x wow for breakfast.

An i7 would be fine, yet so would an E8500 or E8600. The latter offer rediculous bang for your buck and overclock ability.

Seriously, you could twenty box on the rig you have designed there lol.

I have what I consider a "decent" system.
Q9550
8gig GOOD ddr2 1600
HD4890 GPU.
SSD & raid array drives.

I can five box on that rig very easily. 30+ frames in Dalaran, 80+ on main screen out and about with the drone screens limited to 30fps.

A rig similar or even slightly higher that that is all you need for five boxing imo.

But that is just my opinion :P Best of luck with what you end up going with.