RAID can produce some speed advantage over normal disks. Putting your OS on the RAID might speed bootup a bit, but a good chunk of time wasted by the OS is waiting on devices to start, or trying to do a quick new hardware scan. Programs will load quicker off a RAID device, but having more memory is also critical for decent program preformance.

The IDE RAID found on motherboards usually does only support the basic 0, 1, and 0+1. Check out http://anandtech.com/storage/showdoc.html?i=1491 for info on these, and some tests from last year of some IDE cards. Notice the ones that do RAID 5 are much more complex, due to the necessary chips to do the processing of parity. High end SCSI controllers that do 2 types of parity usually have something like a PowerPC chip on them to help out.