Does BI benefit from Dual processor? Dual E5 or a single I7?

I understand how you feel. Building them yourself has a lot of advantages. But usually the price tag beats all and you can't even come close if you build a system yourself versus taking advantage of one of these refurbished systems at their lowest prices. By the time you add the CPU and windows license to your cart, you've already hit the price tag of the (fully warrantied) refurbished system except you haven't added RAM, motherboard, case, power supply, or storage device yet.

Actually, I always upgrade my existing system, which costs a lot less than building a complete new system. I always originally use high-end components, so normally the only thing I have to upgrade is the motherboard and CPU. I use my existing case, modular power supply, graphics card, RAM and the two Blu-ray drives. Next upgrade will be to a 6th-gen motherboard and processor (Skylake).
 
Actually, I always upgrade my existing system, which costs a lot less than building a complete new system. I always originally use high-end components, so normally the only thing I have to upgrade is the motherboard and CPU. I use my existing case, modular power supply, graphics card, RAM and the two Blu-ray drives. Next upgrade will be to a 6th-gen motherboard and processor (Skylake).
You will need likely new memory for skylake, depending on the ddr3 you are using. Even if you simply replace the motherboard and processor, you are looking at about 400 dollars for an i7-4790 system. These can be had with warranty for under 500. Ive seen them as low as 429 from the dell outlet. Buying a complete system also allows you to re-purpose the old one or to donate it.