The Intel ICHxx-R controllers essentially just provide SATA ports. They don't do any of the RAID work by themselves, they rely on drivers which are loaded into the OS when you install it. So yes, they do use your CPU.
The biggest feature of the ICH controllers over a pure software RAID solution is that the controllers include a BIOS which is smart enough to be able to boot from a redundant RAID volume even if one of the discs is dead. So if you have a mirrored system drive and the one designated as the "boot" drive is dead, the RAID controller can boot from the other drive. This is something that a pure software RAID solution can't do.
In terms of the load on the processor, the amount of work that is done for RAID-0 and RAID-1 is basically negligible and can safely be ignored. Even RAID-5 is not a heavy task for modern CPUs, and you only have to do the heavy stuff when writing which is a small percentage of the time in most systems.