The 2 extra SATA ports are standard SATA, and can be used freely, as you want.
Nonetheless, they have a different controller (not the Intel), so you cannot do RAID with both kind. A single RAID should be only on those two, or only on the 6 others, but you can have two raids, one in each controller.
The 6 on Intel have better performance. The other 2 can be used with special software (don't work on Win7 last time I checked) for caching, and accelerating boot time.