Where the Sata III used to have the biggest issue is that it only used one pcie lane theoretically limiting 2-6G-Sata III ports to a combined total of 5G with Marvell's less than stellar efficiency that translated into combined total of about 400MB/s
The new controllers on the x79 boards use two PCIe lanes for a combined total of 10G with Marvell's less than stellar efficiency this is still no where near enough to run both Sata III ports @ full 6G potential but, since it's a shared 10G and not a distributed evenly 10G it's plenty to run 1-Sata III port at (or close to) it's full potential.
The newer Marvell Sata III ports are still less than perfect and are known to have issues in raid as well as the inability to achieve full (or close to full) 6G potential on more than one port at the same time but using only one port isn't an issue.
It's also important to note that I could find no documentation suggesting that this Intel DX79SI had Marvell sata III ports and not Intel sata III ports
But regardless in the review quote I posted the SATA III port got a read speed of 553MB/s and a write speed of 513MB/s essentially maxing out the OCZ Vertex 3
The OP's Sata III Intel 180GB 520 Series SSD has a 550MB/s Read and 520MB/s write so he should have no problem getting close to those speeds.
These same SSD's on the Sata II ports only get 280MB/s Read and 260MB/s write so, to put two of these on the Sata II ports in raid 0 would amount to nothing more than a big waste of money for a (theoretical) improvement of 10MB/s read.