Taken from the Asus web page on the A8N-SLI Premium.
The nFORCE4 chipset incorporated four Serial ATA and two parallel connectors with high performance RAID functions in RAID 0, RAID 1, RAID 0+1 and JBOD. The Silicon Image controller provides another four Serial ATA connectors for RAID 0, RAID 1, RAID1+0, and RAID 5 functions. The A8N-SLI Premium is the ideal solution to enhance hard disk performance and data back up protection without the cost of add-on cards.
Does this mean that the NV SATA ports are connected directly to the Southbridge and have a larger potential top speed in a RAID set up than the Silicon Image ports?