Much of the RAID's CPU usage will be drawn against processes that actually perform the I/O -- such is the nature of a multi-threaded monolithic style OS.

But Linux s/w RAID does indeed outperform nearly(?) all h/w RAID solutions, so it's kinda hard to go wrong by it. Especially if combined with some form of hotswap capable hardware (SATA, Firewire, USB2, etc..).

Cheers