If performance is a requirement, then you would be best serverd to get a Hardware RAID controller.
Check 3Ware for Hardware controllers that are well supported under Linux. http://www.3ware.com/products/serial_ata.asp
Odds are your onboard controllers don't support features such as NCQ (Native Command Queueing
Hot swap support
Online array expansion
lower CPU Utilization
etc.. etc.. etc..
There are other benefits of a hardware based solution.
So frankly for an array that size I wouldn't mess with Software based RAID.