Why the i7. I'm sure you read this.
"Our tests demonstrate fairly little difference between a $225 LGA 1155 Core i5-2500K and a $1000 LGA 2011 Core i7-3960X, even when three-way graphics card configurations are involved. It turns out that memory bandwidth and PCIe throughput don't hold back the performance of existing Sandy Bridge-based machines. "
A long time ago tests changing motherboards yielded single digit performance changes. Rarely pays off.
The same thing with OS changes but 7 does have an edge on compatibility for now.