I have a Tyan Transport GX28 with the S2881 motherboard, which sports the same RAID controller. When I was getting FreeBSD running on it, RAID was a no-go. Since then,
ataraid(4) has added support for writing SiS metadata. I cannot say anything to the worthwhileness of using it.
On a side note, if that motherboard has Broadcom ethernet onboard, and you experience random lock ups, disable the onboard ethernet via the jumpers. Not using the interface or not loading the drivers are not enough to prevent the lockups (based on my experience).
Caveat: this was all in FreeBSD 6.0, so things (such as SiS metadata support in ataraid) may have changed.