For indexer requirements, the following is listed as the recommendation configuration in the Planning Your Splunk Deployment Guide:
8 300GB SAS hard disks at 10,000 rpm each in RAID 10 * capable of 1200 IO operations / second (Iopps)
Does this recommendation change when using software vs. hardware RAID? If so, how?
The only change with soft raid is that you may want to invest a little more in CPU than you might with hardware raid, especially if you are considering using raid 5 (which we obviously recommend against for the warm buckets). The read and write rates of Splunk are typically more bursty than sustained, so the CPU increase is not likely to be very large.
Software raid is typically a higher total performance solution, because the bandwidth of commodity processors grows much faster than custom raid solutions. The typical stumbling block is the boot behavior on disk failure. That's general IT stuff though, not terribly Splunk-specific.