Because Intel is either dumb or greedy wanting to sell it's DX79SR for 30 dollars more with its 2 extra SATA 6 ports, are the DX79SI's 2 SATA 6 ports and 4 SATA 3 ports good enough for 2x SSD in RAID 0(if that board even supports it) and 2x 2TB Caviar black HDD/Seagate Barracuda in 2 of the SATA 3 ports.
BTW why do people say HDD in SATA 6 makes no difference vs SATA 3 when a lot of hard drives on Newegg are reccomended for SATA 6 in the description? Who's wrong here?
1. you will get little gains from putting ssds in raid. two will bottleneck a sata 3 port.
2. unless you are planning to use more then 2 ssds you will not need more then 2 sata 3 ports.
3. a typical hard drive will not be fast enough to show performance gains on a sata 3 port.
Only a modern ssd can saturate a sata 2 port, that is why sata 3 is recommended for attaching ssd's.
ssd's in raid-0 do not give you any more performance. In benchmarks, yes, in actual usage, you can't tell the difference.
And... unless you have a Z77 based chipset, you will lose "trim" support in raid-0.
It is better to buy a ssd that is twice as large; they perform a bit faster.
Modern hard drives do support sata 3. But that is only for transferring the contents of the cache or buffer to the port. That is a very minor component of performance.
It is the mechanical performance that is the limiting factor on hard drives. So, as a practical point of view, sata 2/3 attachment makes no difference except for SSD's.